Add null check when accessing WebLocalFrameImpl::mainFrameImpl().

Source/web/WebViewImpl.cpp

 /*
  * Copyright (C) 2011, 2012 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 424 matching lines @@
     m_inspectorSettingsMap = adoptPtr(new SettingsMap);
 }
 
 WebViewImpl::~WebViewImpl()
 {
     ASSERT(!m_page);
 }
 
 WebLocalFrameImpl* WebViewImpl::mainFrameImpl()
 {
-    return m_page && m_page->mainFrame()->isLocalFrame() ? WebLocalFrameImpl::fromFrame(m_page->deprecatedLocalMainFrame()) : 0;
+    return m_page && m_page->mainFrame() && m_page->mainFrame()->isLocalFrame() ? WebLocalFrameImpl::fromFrame(m_page->deprecatedLocalMainFrame()) : 0;
 }
 
 bool WebViewImpl::tabKeyCyclesThroughElements() const
 {
     ASSERT(m_page);
     return m_page->tabKeyCyclesThroughElements();
 }
 
 void WebViewImpl::setTabKeyCyclesThroughElements(bool value)
 {
@@ skipping 3681 matching lines @@
     const PageScaleConstraints& constraints = m_pageScaleConstraintsSet.pageDefinedConstraints();
 
     if (!mainFrameImpl() || !mainFrameImpl()->frameView())
         return false;
 
     return mainFrameImpl()->frameView()->layoutSize().width() == m_size.width
         || (constraints.minimumScale == constraints.maximumScale && constraints.minimumScale != -1);
 }
 
 } // namespace blink