Allow capturing transparent screenshots (#354)

This patch adds `omitBackground` option to the `page.screenshot` method.

Fixes #344

docs/api.md

@@ -604,6 +604,7 @@ Shortcut for [`keyboard.down`](#keyboarddownkey-options) and [`keyboard.up`](#ke
         - `y` <[number]> y-coordinate of top-left corner of clip area
         - `width` <[number]> width of clipping area
         - `height` <[number]> height of clipping area
+    - `omitBackground` <[boolean]> Hides default white background and allows capturing screenshots with transparency. Defaults to `false`.
 - returns: <[Promise]<[Buffer]>> Promise which resolves to buffer with captured screenshot
 
 #### page.setContent(html)

lib/Page.js

@@ -467,7 +467,11 @@ class Page extends EventEmitter {
       await this._client.send('Emulation.setDeviceMetricsOverride', { mobile, width, height, deviceScaleFactor, screenOrientation });
     }
 
+    if (options.omitBackground)
+      await this._client.send('Emulation.setDefaultBackgroundColorOverride', { color: { r: 0, g: 0, b: 0, a: 0 } });
     let result = await this._client.send('Page.captureScreenshot', { format, quality: options.quality, clip });
+    if (options.omitBackground)
+      await this._client.send('Emulation.setDefaultBackgroundColorOverride');
 
     if (options.fullPage)
       await this.setViewport(this._viewport);

test/test.js

@@ -1810,6 +1810,12 @@ describe('Page', function() {
         expect(screenshots[i]).toBeGolden(`grid-cell-${i}.png`);
       await Promise.all(pages.map(page => page.close()));
     }));
+    it('should allow transparency', SX(async function() {
+      await page.setViewport({ width: 100, height: 100 });
+      await page.goto(EMPTY_PAGE);
+      let screenshot = await page.screenshot({omitBackground:true});
+      expect(screenshot).toBeGolden('transparent.png');
+    }));
   });
 
   describe('Tracing', function() {