DevTools: make paint profiler target-aware

Source/devtools/front_end/common/utilities.js

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2012 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 1759 matching lines @@
             this._outgoingCallback();
     }
 }
 
 /**
  * @param {*} value
  */
 function suppressUnused(value)
 {
 }
+
+/**
+ * @constructor
+ * @param {!T} targetObject
+ * @template T
+ */
+function WeakReference(targetObject)
+{
+    this._targetObject = targetObject;
+}
+
+WeakReference.prototype = {
+    /**
+     * @return {?T}
+     */
+    get: function()
+    {
+        return this._targetObject;
+    },
+
+    clear: function()
+    {
+        this._targetObject = null;
+    }
+};
+