Implement FrameManager

This patch implements FrameManager which is responsible for maintaining
the frame tree. FrameManager is quite basic: it sends FrameAttached,
FrameDetached and FrameNavigated events, and can report mainFrame and
all frames.

The next step would be moving certain Page API's to the Frame. For
example, such method as Page.evaluate, Page.navigate and others should
be available on Frame object as well.

References #4

lib/Page.js

@@ -20,6 +20,7 @@ var helpers = require('./helpers');
 var mime = require('mime');
 var Request = require('./Request');
 var Dialog = require('./Dialog');
+var FrameManager = require('./FrameManager');
 
 class Page extends EventEmitter {
     /**
@@ -35,7 +36,8 @@ class Page extends EventEmitter {
         ]);
         var expression = helpers.evaluationString(() => window.devicePixelRatio, []);
         var {result:{value: screenDPI}} = await client.send('Runtime.evaluate', { expression, returnByValue: true });
-        var page = new Page(client, screenDPI);
+        var frameManager = await FrameManager.create(client);
+        var page = new Page(client, frameManager, screenDPI);
         // Initialize default page size.
         await page.setViewportSize({width: 400, height: 300});
         return page;
@@ -43,11 +45,13 @@ class Page extends EventEmitter {
 
     /**
      * @param {!Connection} client
+     * @param {!FrameManager} frameManager
      * @param {number} screenDPI
      */
-    constructor(client, screenDPI) {
+    constructor(client, frameManager, screenDPI) {
         super();
         this._client = client;
+        this._frameManager = frameManager;
         this._screenDPI = screenDPI;
         this._extraHeaders = {};
         /** @type {!Map<string, function>} */
@@ -57,6 +61,10 @@ class Page extends EventEmitter {
 
         this._screenshotTaskChain = Promise.resolve();
 
+        this._frameManager.on(FrameManager.Events.FrameAttached, event => this.emit(Page.Events.FrameAttached, event));
+        this._frameManager.on(FrameManager.Events.FrameDetached, event => this.emit(Page.Events.FrameDetached, event));
+        this._frameManager.on(FrameManager.Events.FrameNavigated, event => this.emit(Page.Events.FrameNavigated, event));
+
         client.on('Network.responseReceived', event => this.emit(Page.Events.ResponseReceived, event.response));
         client.on('Network.loadingFailed', event => this.emit(Page.Events.ResourceLoadingFailed, event));
         client.on('Network.requestIntercepted', event => this._onRequestIntercepted(event));
@@ -65,6 +73,20 @@ class Page extends EventEmitter {
         client.on('Runtime.exceptionThrown', exception => this._handleException(exception.exceptionDetails));
     }
 
+    /**
+     * @return {!Frame}
+     */
+    mainFrame() {
+        return this._frameManager.mainFrame();
+    }
+
+    /**
+     * @return {!Array<!Frame>}
+     */
+    frames() {
+        return this._frameManager.frames();
+    }
+
     /**
      * @param {?function(!Request)} interceptor
      */
@@ -583,6 +605,9 @@ Page.Events = {
     Error: 'error',
     ResourceLoadingFailed: 'resourceloadingfailed',
     ResponseReceived: 'responsereceived',
+    FrameAttached: 'frameattached',
+    FrameDetached: 'framedetached',
+    FrameNavigated: 'framenavigated',
 };
 
 module.exports = Page;