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

Source/core/frame/RemoteFrameView.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 RemoteFrameView_h
 #define RemoteFrameView_h
 
 #include "platform/Widget.h"
 #include "platform/geometry/IntRect.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class RemoteFrame;
 
 class RemoteFrameView : public Widget {
 public:
     static PassRefPtr<RemoteFrameView> create(RemoteFrame*);
 
     virtual ~RemoteFrameView();
 
     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;
 
     virtual void invalidateRect(const IntRect&) OVERRIDE;
 
     virtual void setFrameRect(const IntRect&) OVERRIDE;
 
 private:
     explicit RemoteFrameView(RemoteFrame*);
 
     // The RefPtrWillBePersistent-cycle between RemoteFrame and its RemoteFrameView
     // is broken in the same manner as FrameView::m_frame and LocalFrame::m_view is.
     // See the FrameView::m_frame comment.
     RefPtrWillBePersistent<RemoteFrame> m_remoteFrame;
 };
 
 DEFINE_TYPE_CASTS(RemoteFrameView, Widget, widget, widget->isRemoteFrameView(), widget.isRemoteFrameView());
 
 } // namespace blink
 
 #endif // RemoteFrameView_h