Add Blink APIs for frame tree mirroring.

Source/core/frame/FrameOwner.h

Index: Source/core/frame/FrameOwner.h
diff --git a/Source/core/frame/FrameOwner.h b/Source/core/frame/FrameOwner.h
index 7b1f891402c6ac56408f1eacb910e4a3412110b4..749e69c52c584602565d540e4a16849c2191c3a8 100644
--- a/Source/core/frame/FrameOwner.h
+++ b/Source/core/frame/FrameOwner.h
@@ -9,12 +9,21 @@
 
 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