DevTools: Add user metric for CPU throttling enable.

third_party/WebKit/Source/devtools/front_end/components/CPUThrottlingManager.js

 // Copyright 2017 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.
 
 /**
  * @implements {SDK.TargetManager.Observer}
  */
 Components.CPUThrottlingManager = class extends Common.Object {
   constructor() {
     super();
     this._throttlingRate = 1;  // No throttling
     SDK.targetManager.observeTargets(this, SDK.Target.Capability.Browser);
     /** @type {!Set<!UI.ToolbarComboBox>} */
     this._controls = new Set();
     this._rates = [1, 2, 5, 10, 20];
   }
 
   /**
    * @param {number} index
    */
   _setRateIndex(index) {
     this._throttlingRate = this._rates[index];
     SDK.targetManager.targets().forEach(target => target.emulationAgent().setCPUThrottlingRate(this._throttlingRate));
     var icon = null;
     if (this._throttlingRate !== 1) {
+      Host.userMetrics.actionTaken(Host.UserMetrics.Action.CpuThrottlingEnabled);
       icon = UI.Icon.create('smallicon-warning');
       icon.title = Common.UIString('CPU throttling is enabled');
     }
     for (var control of this._controls)
       control.setSelectedIndex(index);
     UI.inspectorView.setPanelIcon('timeline', icon);
     this.dispatchEventToListeners(Components.CPUThrottlingManager.Events.RateChanged);
   }
 
   /**
@@ skipping 43 matching lines @@
    */
   disposeControl(control) {
     this._controls.delete(control);
   }
 };
 
 /** @enum {symbol} */
 Components.CPUThrottlingManager.Events = {
   RateChanged: Symbol('RateChanged')
 };