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

Source/core/testing/DummyPageHolder.cpp

 /*
  * Copyright (c) 2013, Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 78 matching lines @@
     m_frame.clear();
 }
 
 Page& DummyPageHolder::page() const
 {
     return *m_page;
 }
 
 LocalFrame& DummyPageHolder::frame() const
 {
+    ASSERT(m_frame);
     return *m_frame;
 }
 
 FrameView& DummyPageHolder::frameView() const
 {
     return *m_frame->view();
 }
 
 Document& DummyPageHolder::document() const
 {
     return *m_frame->domWindow()->document();
 }
 
 } // namespace blink