DevTools: use target-based model accessors only.

Source/devtools/front_end/timeline/TimelinePowerGraph.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.CountersGraph}
  * @implements {WebInspector.TimelineModeView}
  * @param {!WebInspector.TimelineModeViewDelegate} delegate
  * @param {!WebInspector.TimelineModel} model
  */
 WebInspector.TimelinePowerGraph = function(delegate, model)
 {
     WebInspector.CountersGraph.call(this, WebInspector.UIString("POWER"), delegate, model);
 
     this._counter = this.createCounter(WebInspector.UIString("Power"), WebInspector.UIString("Power: %.2f\u2009watts"), "#d00");
-    WebInspector.powerProfiler.addEventListener(WebInspector.PowerProfiler.EventTypes.PowerEventRecorded, this._onRecordAdded, this);
+    this.target().powerProfiler.addEventListener(WebInspector.PowerProfiler.EventTypes.PowerEventRecorded, this._onRecordAdded, this);
 }
 
 WebInspector.TimelinePowerGraph.prototype = {
     /**
      * @override
      */
     dispose: function()
     {
         WebInspector.CountersGraph.prototype.dispose.call(this);
-        WebInspector.powerProfiler.removeEventListener(WebInspector.PowerProfiler.EventTypes.PowerEventRecorded, this._onRecordAdded, this);
+        this.target().powerProfiler.removeEventListener(WebInspector.PowerProfiler.EventTypes.PowerEventRecorded, this._onRecordAdded, this);
     },
 
     _onRecordAdded: function(event)
     {
         var record = event.data;
         if (!this._previousRecord) {
             this._previousRecord = record;
             return;
         }
 
         // "value" of original PowerEvent means the average power between previous sampling to current one.
         // Here, it is converted to average power between current sampling to next one.
         this._counter.appendSample(this._previousRecord.timestamp, record.value);
         this._previousRecord = record;
         this.scheduleRefresh();
     },
 
     __proto__: WebInspector.CountersGraph.prototype
 }