Implement Page.evaluateOnInitilized method

The PhantomJS has a similar callback called onInitialized. This
callback passes control to the automation script when the page
gets initialized.

To precisely implement this functionality atop of puppeteer,
and since puppeteer controller script lives in a separate process to
the page, we need an ability to pause page at the moment of
initialization. For now, we are not able to do this.

However, oftentimes clients want to evaluate certain code in
page at the point of page initialization. This patch implements
this capability with the Page.evaluateOnInitilized method call.

This patch also re-implements phantom's unrandomize.js example
with the puppeteer API. This is serves an illustration purpose
for the page.evaluateOnInitilized callback.

examples/unrandomize.js

@@ -0,0 +1,33 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+var Browser = require('../lib/Browser');
+var browser = new Browser();
+
+browser.newPage().then(async page => {
+    await page.evaluateOnInitialized(function() {
+        Math.random = () => 42 / 100;
+    });
+    var result = await page.navigate('http://ariya.github.com/js/random/');
+    if (result) {
+        console.log(await page.evaluate(function () {
+            return document.getElementById('numbers').textContent;
+        }));
+    } else {
+        console.log('Network error.');
+    }
+    browser.close();
+});

lib/Page.js

@@ -304,6 +304,18 @@ class Page extends EventEmitter {
         return response.result.value;
     }
 
+    /**
+     * @param {function()} fun
+     * @param {!Array<*>} args
+     * @return {!Promise}
+     */
+    async evaluateOnInitialized(fun, ...args) {
+        var code = helpers.evaluationString(fun, args, false);
+        await this._client.send('Page.addScriptToEvaluateOnLoad', {
+            scriptSource: code
+        });
+    }
+
     /**
      * @param {!Page.ScreenshotType} screenshotType
      * @param {?{x: number, y: number, width: number, height: number}} clipRect

lib/helpers.js

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-module.exports = {
+var helpers = module.exports = {
     /**
      * @param {!CDP} client
      * @param {string} url
@@ -53,16 +53,28 @@ module.exports = {
      * @return {!Promise<!Object>}
      */
     evaluate: function(client, fun, args, awaitPromise, sourceURL) {
-        var argsString = args.map(x => JSON.stringify(x)).join(',');
-        var code = `(${fun.toString()})(${argsString})`;
-        if (awaitPromise)
-            code = `Promise.resolve(${code})`;
-        if (sourceURL)
-            code += `\n//# sourceURL=${sourceURL}`;
+        var code = helpers.evaluationString(fun, args, awaitPromise, sourceURL);
         return client.send('Runtime.evaluate', {
             expression: code,
             awaitPromise: awaitPromise,
             returnByValue: true
         });
     },
+
+    /**
+     * @param {function()} fun
+     * @param {!Array<*>} args
+     * @param {boolean} awaitPromise
+     * @param {string=} sourceURL
+     * @return {string}
+     */
+    evaluationString: function(fun, args, awaitPromise, sourceURL) {
+        var argsString = args.map(x => JSON.stringify(x)).join(',');
+        var code = `(${fun.toString()})(${argsString})`;
+        if (awaitPromise)
+            code = `Promise.resolve(${code})`;
+        if (sourceURL)
+            code += `\n//# sourceURL=${sourceURL}`;
+        return code;
+    }
 }