DevTools: use target-based model accessors only.

Source/devtools/front_end/sdk/DebuggerModel.js

 /*
  * Copyright (C) 2010 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 1172 matching lines @@
      */
     exception: function()
     {
         if (this.reason !== WebInspector.DebuggerModel.BreakReason.Exception && this.reason !== WebInspector.DebuggerModel.BreakReason.PromiseRejection)
             return null;
         return this.target().runtimeModel.createRemoteObject(/** @type {!RuntimeAgent.RemoteObject} */(this.auxData));
     },
 
     __proto__: WebInspector.SDKObject.prototype
 }
-
-/**
- * @type {!WebInspector.DebuggerModel}
- */
-WebInspector.debuggerModel;