Add Blink APIs for frame tree mirroring.

Source/core/frame/FrameOwner.h

 // 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.
 
 #ifndef FrameOwner_h
 #define FrameOwner_h
 
 #include "core/dom/SandboxFlags.h"
 
 namespace WebCore {
 
+class Frame;
+
 class FrameOwner {
 public:
     virtual bool isLocal() const = 0;
 
+    virtual void setContentFrame(Frame&) = 0;
+    // clearContentFrame() may result in the destruction of the FrameOwner.
+    virtual void clearContentFrame() = 0;
+
     virtual SandboxFlags sandboxFlags() const = 0;
     virtual void dispatchLoad() = 0;
+
+protected:
+    virtual ~FrameOwner() { }
 };
 
 } // namespace WebCore
 
 #endif // FrameOwner_h