Timeline: produce session cookie when timeline is started

Source/devtools/front_end/TracingModel.js

 /*
  * Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 /**
  * @constructor
  * @extends {WebInspector.Object}
  */
@@ skipping 45 matching lines @@
     CreateObject: "N",
     SnapshotObject: "O",
     DeleteObject: "D"
 };
 
 WebInspector.TracingModel.MetadataEvent = {
     ProcessSortIndex: "process_sort_index",
     ProcessName: "process_name",
     ThreadSortIndex: "thread_sort_index",
     ThreadName: "thread_name"
-}
+};
+
+WebInspector.TracingModel.DevToolsMetadataEventCategory = "disabled-by-default-devtools.timeline";
+
+WebInspector.TracingModel.DevToolsMetadataEvent = {
+    TimelineStarted: "TimelineStarted",
+    SetLayerTreeId: "SetLayerTreeId"
+};
 
 WebInspector.TracingModel.prototype = {
     /**
      * @param {string} categoryPatterns
      * @param {string} options
      * @param {function(?string)=} callback
      */
     start: function(categoryPatterns, options, callback)
     {
         this.reset();
@@ skipping 364 matching lines @@
     dataCollected: function(data)
     {
         this._tracingModel._eventsCollected(data);
     },
 
     tracingComplete: function()
     {
         this._tracingModel._tracingComplete();
     }
 }