Fix Frame::isMainFrame() to be compatible with provisional frames

Source/web/WebRemoteFrameImpl.cpp

 // 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.
 
 #include "config.h"
 #include "web/WebRemoteFrameImpl.h"
 
 #include "core/frame/FrameView.h"
 #include "core/frame/RemoteFrame.h"
 #include "core/frame/Settings.h"
@@ skipping 175 matching lines @@
 }
 
 void WebRemoteFrameImpl::removeChild(WebFrame* frame)
 {
     WebFrame::removeChild(frame);
     m_ownersForChildren.remove(frame);
 }
 
 WebDocument WebRemoteFrameImpl::document() const
 {
+    // TODO(dcheng): this should also ASSERT_NOT_REACHED, but a lot of
+    // code tries to access the document of a remote frame at the moment.
     return WebDocument();
 }
 
 WebPerformance WebRemoteFrameImpl::performance() const
 {
     ASSERT_NOT_REACHED();
     return WebPerformance();
 }
 
 bool WebRemoteFrameImpl::dispatchBeforeUnloadEvent()
@@ skipping 602 matching lines @@
 {
     frame()->setIsLoading(false);
     if (parent() && parent()->isWebLocalFrame()) {
         WebLocalFrameImpl* parentFrame =
             toWebLocalFrameImpl(parent()->toWebLocalFrame());
         parentFrame->frame()->loader().checkCompleted();
     }
 }
 
 } // namespace blink