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

Source/web/WebRemoteFrameImpl.cpp

Index: Source/web/WebRemoteFrameImpl.cpp
diff --git a/Source/web/WebRemoteFrameImpl.cpp b/Source/web/WebRemoteFrameImpl.cpp
index 22ea043eb00c4b8850a27461a2aa8c0fda35c61a..0d2a4d42f7a225c200615ddf46e96d9e766bc6be 100644
--- a/Source/web/WebRemoteFrameImpl.cpp
+++ b/Source/web/WebRemoteFrameImpl.cpp
@@ -193,6 +193,8 @@ void WebRemoteFrameImpl::removeChild(WebFrame* 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();
 }