DevTools: Add user metric for CPU throttling enable.

third_party/WebKit/Source/devtools/front_end/host/UserMetrics.js

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 65 matching lines @@
   AnimationsPlaybackRateChanged: 11,
   RevisionApplied: 12,
   FileSystemDirectoryContentReceived: 13,
   StyleRuleEdited: 14,
   CommandEvaluatedInConsolePanel: 15,
   DOMPropertiesExpanded: 16,
   ResizedViewInResponsiveMode: 17,
   TimelinePageReloadStarted: 18,
   ConnectToNodeJSFromFrontend: 19,
   ConnectToNodeJSDirectly: 20,
+  CpuThrottlingEnabled: 21,
 };
 
 Host.UserMetrics._PanelCodes = {
   elements: 1,
   resources: 2,
   network: 3,
   sources: 4,
   timeline: 5,
   heap_profiler: 6,
   audits: 7,
   console: 8,
   layers: 9,
   'drawer-console': 10,
   'drawer-animations': 11,
   'drawer-network.config': 12,
   'drawer-rendering': 13,
   'drawer-sensors': 14,
   'drawer-sources.search': 15,
   security: 16,
   js_profiler: 17
 };
 
 /** @type {!Host.UserMetrics} */
 Host.userMetrics = new Host.UserMetrics();