Assert that frame() derefs a non-null pointer.

Source/core/frame/RemoteFrameView.h

Index: Source/core/frame/RemoteFrameView.h
diff --git a/Source/core/frame/RemoteFrameView.h b/Source/core/frame/RemoteFrameView.h
index b6b29050b887361a31f2e7ed305637eb037d28b9..ecce68ad11f96c4dce692c9b5cf37d8396051de6 100644
--- a/Source/core/frame/RemoteFrameView.h
+++ b/Source/core/frame/RemoteFrameView.h
@@ -21,7 +21,11 @@ public:
 
     virtual bool isRemoteFrameView() const OVERRIDE { return true; }
 
-    RemoteFrame& frame() const { return *m_remoteFrame; }
+    RemoteFrame& frame() const
+    {
+        ASSERT(m_remoteFrame);
+        return *m_remoteFrame;
+    }
 
     // Override to notify remote frame that its viewport size has changed.
     virtual void frameRectsChanged() OVERRIDE;