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

Source/core/editing/InputMethodController.h

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 89 matching lines @@
     // We don't use PlainTextRange which is immutable, for composition range.
     unsigned m_compositionStart;
     unsigned m_compositionEnd;
     // startOffset and endOffset of CompositionUnderline are based on
     // m_compositionNode.
     Vector<CompositionUnderline> m_customCompositionUnderlines;
 
     explicit InputMethodController(LocalFrame&);
 
     Editor& editor() const;
-    LocalFrame& frame() const { return *m_frame; }
+    LocalFrame& frame() const
+    {
+        ASSERT(m_frame);
+        return *m_frame;
+    }
 
     bool insertTextForConfirmedComposition(const String& text);
     void selectComposition() const;
     enum FinishCompositionMode { ConfirmComposition, CancelComposition };
     // Returns true if composition exists.
     bool finishComposition(const String&, FinishCompositionMode);
     bool setSelectionOffsets(const PlainTextRange&);
 };
 
 } // namespace blink
 
 #endif // InputMethodController_h