[DevTools] Add user metric action DeviceModeEnabled.

Source/devtools/front_end/main/AdvancedApp.js

Index: Source/devtools/front_end/main/AdvancedApp.js
diff --git a/Source/devtools/front_end/main/AdvancedApp.js b/Source/devtools/front_end/main/AdvancedApp.js
index 6480e55fb414a6479efae174ac6c0f028a499665..97b4c41c73764c46288d2b978292b05ea60d3954 100644
--- a/Source/devtools/front_end/main/AdvancedApp.js
+++ b/Source/devtools/front_end/main/AdvancedApp.js
@@ -22,7 +22,10 @@ WebInspector.AdvancedApp = function()
 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()