Implement printing to PDF

This patch implements:
- puppeteer's Page.printToPDF method. The method's defaults
  are similar to phantom's defaults for the paperSize property.
- phantom's render into pdf file.

lib/Page.js

@@ -386,6 +386,54 @@ class Page extends EventEmitter {
         fs.writeFileSync(filePath, buffer);
     }
 
+    /**
+     * @param {string} filePath
+     * @param {!Object=} options
+     */
+    async printToPDF(filePath, options) {
+        options = options || {};
+
+        var scale = options.scale || 1;
+        var displayHeaderFooter = options.displayHeaderFooter || false;
+        var printBackground = options.printBackground || true;
+        var landscape = options.landscape || false;
+        var pageRanges = options.pageRanges || '';
+
+        var paperWidth = 8.5;
+        var paperHeight = 11;
+        if (options.format) {
+            var format = Page.PaperFormats[options.format];
+            console.assert(format, 'Unknown paper format: ' + options.format);
+            paperWidth = format.width;
+            paperHeight = format.height;
+        } else {
+            paperWidth = convertPrintParameterToInches(options.width) || paperWidth;
+            paperHeight = convertPrintParameterToInches(options.height) || paperHeight;
+        }
+
+        var marginOptions = options.margin || {};
+        var marginTop = convertPrintParameterToInches(marginOptions.top) || 0;
+        var marginLeft = convertPrintParameterToInches(marginOptions.left) || 0;
+        var marginBottom = convertPrintParameterToInches(marginOptions.bottom) || 0;
+        var marginRight = convertPrintParameterToInches(marginOptions.right) || 0;
+
+        var result = await this._client.send('Page.printToPDF', {
+            landscape: landscape,
+            displayHeaderFooter: displayHeaderFooter,
+            printBackground: printBackground,
+            scale: scale,
+            paperWidth: paperWidth,
+            paperHeight: paperHeight,
+            marginTop: marginTop,
+            marginBottom: marginBottom,
+            marginLeft: marginLeft,
+            marginRight: marginRight,
+            pageRanges: pageRanges
+        });
+        var buffer = new Buffer(result.data, 'base64');
+        fs.writeFileSync(filePath, buffer);
+    }
+
     /**
      * @return {!Promise<string>}
      */
@@ -429,6 +477,59 @@ Page.ScreenshotTypes = {
     JPG: "jpeg",
 };
 
+/** @enum {string} */
+Page.PaperFormats = {
+    Letter: {width: 8.5, height: 11},
+    Legal: {width: 8.5, height: 14},
+    Tabloid: {width: 11, height: 17},
+    Ledger: {width: 17, height: 11},
+    A0: {width: 33.1, height: 46.8 },
+    A1: {width: 23.4, height: 33.1 },
+    A2: {width: 16.5, height: 23.4 },
+    A3: {width: 11.7, height: 16.5 },
+    A4: {width: 8.27, height: 11.7 },
+    A5: {width: 5.83, height: 8.27 },
+};
+
+var unitToPixels = {
+    'px': 1,
+    'in': 96,
+    'cm': 37.8,
+    'mm': 3.78
+};
+
+/**
+ * @param {(string|number|undefined)} parameter
+ * @return {(number|undefined)}
+ */
+function convertPrintParameterToInches(parameter) {
+    if (typeof parameter === 'undefined')
+        return undefined;
+    var pixels;
+    if (typeof parameter === 'number') {
+        // Treat numbers as pixel values to be aligned with phantom's paperSize.
+        pixels = /** @type {number} */ (parameter);
+    } else if (typeof parameter === 'string') {
+        var text = parameter;
+        var unit = text.substring(text.length - 2).toLowerCase();
+        var valueText = '';
+        if (unitToPixels.hasOwnProperty(unit)) {
+            valueText = text.substring(0, text.length - 2);
+        } else {
+            // In case of unknown unit try to parse the whole parameter as number of pixels.
+            // This is consistent with phantom's paperSize behavior.
+            unit = 'px';
+            valueText = text;
+        }
+        var value = Number(valueText);
+        console.assert(!isNaN(value), 'Failed to parse parameter value: ' + text);
+        var pixels = value * unitToPixels[unit];
+    } else {
+        throw new Error('printToPDF Cannot handle parameter type: ' + (typeof parameter));
+    }
+    return pixels / 96;
+};
+
 Page.Events = {
     Alert: 'alert',
     BeforeUnload: 'beforeunload',

phantomjs/WebPage.js

@@ -276,7 +276,18 @@ class WebPage {
                 height: this.clipRect.height
             };
         }
-        var imageBuffer = await(this._page.saveScreenshot(fileName, clipRect));
+        if (fileName.endsWith('pdf')) {
+            var options = {};
+            var paperSize = this.paperSize || {};
+            options.margin = paperSize.margin;
+            options.format = paperSize.format;
+            options.landscape = paperSize.orientation === 'landscape';
+            options.width = paperSize.width;
+            options.height = paperSize.height;
+            await(this._page.printToPDF(fileName, options));
+        } else {
+            await(this._page.saveScreenshot(fileName, clipRect));
+        }
     }
 
     release() {