[DevTools] Add user metric action DeviceModeEnabled.

Source/devtools/front_end/main/AdvancedApp.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.App}
  */
 WebInspector.AdvancedApp = function()
 {
     WebInspector.App.call(this);
     if (WebInspector.overridesSupport.responsiveDesignAvailable()) {
         this._toggleEmulationButton = new WebInspector.StatusBarButton(WebInspector.UIString("Toggle device mode."), "emulation-status-bar-item");
         this._toggleEmulationButton.toggled = WebInspector.overridesSupport.emulationEnabled();
         this._toggleEmulationButton.addEventListener("click", this._toggleEmulationEnabled, this);
         WebInspector.overridesSupport.addEventListener(WebInspector.OverridesSupport.Events.EmulationStateChanged, this._emulationEnabledChanged, this);
         WebInspector.overridesSupport.addEventListener(WebInspector.OverridesSupport.Events.OverridesWarningUpdated, this._overridesWarningUpdated, this);
     }
     WebInspector.dockController.addEventListener(WebInspector.DockController.Events.BeforeDockSideChanged, this._openToolboxWindow, this);
 };
 
 WebInspector.AdvancedApp.prototype = {
     _toggleEmulationEnabled: function()
     {
-        WebInspector.overridesSupport.setEmulationEnabled(!this._toggleEmulationButton.toggled);
+        var enabled = !this._toggleEmulationButton.toggled;
+        if (enabled)
+            WebInspector.userMetrics.DeviceModeEnabled.record();
+        WebInspector.overridesSupport.setEmulationEnabled(enabled);
     },
 
     _emulationEnabledChanged: function()
     {
         this._toggleEmulationButton.toggled = WebInspector.overridesSupport.emulationEnabled();
         if (!WebInspector.overridesSupport.responsiveDesignAvailable() && WebInspector.overridesSupport.emulationEnabled())
             WebInspector.inspectorView.showViewInDrawer("emulation", true);
     },
 
     _overridesWarningUpdated: function()
@@ skipping 269 matching lines @@
     handleAction: function()
     {
         if (!WebInspector.overridesSupport.responsiveDesignAvailable())
             return false;
         if (!(WebInspector.app instanceof WebInspector.AdvancedApp))
             return false;
         WebInspector.app._toggleEmulationEnabled();
         return true;
     }
 }