Have Node::document() return a reference instead of a pointer

Source/web/ChromeClientImpl.cpp

Index: Source/web/ChromeClientImpl.cpp
diff --git a/Source/web/ChromeClientImpl.cpp b/Source/web/ChromeClientImpl.cpp
index b46972ffb2d6ffffa88ae66ae3c0dc34cf087371..ac4d65aa7aaeb3b359746d3cbe096126fbbcc726 100644
--- a/Source/web/ChromeClientImpl.cpp
+++ b/Source/web/ChromeClientImpl.cpp
@@ -759,7 +759,7 @@ void ChromeClientImpl::formStateDidChange(const Node* node)
 
     // The current history item is not updated yet.  That happens lazily when
     // WebFrame::currentHistoryItem is requested.
-    WebFrameImpl* webframe = WebFrameImpl::fromFrame(node->document()->frame());
+    WebFrameImpl* webframe = WebFrameImpl::fromFrame(node->document().frame());
     if (webframe->client())
         webframe->client()->didUpdateCurrentHistoryItem(webframe);
 }