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

Source/core/frame/FrameView.h

Index: Source/core/frame/FrameView.h
diff --git a/Source/core/frame/FrameView.h b/Source/core/frame/FrameView.h
index 983a67b3dc762e96b3d64d70317cb4bf68c19e6b..c6eb7af5c550811603d608d718e02af1a2d39dce 100644
--- a/Source/core/frame/FrameView.h
+++ b/Source/core/frame/FrameView.h
@@ -72,7 +72,12 @@ public:
     virtual void invalidateRect(const IntRect&) OVERRIDE;
     virtual void setFrameRect(const IntRect&) OVERRIDE;
 
-    LocalFrame& frame() const { return *m_frame; }
+    LocalFrame& frame() const
+    {
+        ASSERT(m_frame);
+        return *m_frame;
+    }
+
     Page* page() const;
 
     RenderView* renderView() const;