Have Frame::tree() return a reference

Source/web/StorageAreaProxy.cpp

Index: Source/web/StorageAreaProxy.cpp
diff --git a/Source/web/StorageAreaProxy.cpp b/Source/web/StorageAreaProxy.cpp
index fccadd748ed9bf8814f202391b525312d143d0a2..b3b8ed7307ccabf9ad5407fa0c15d25e55b7a9e0 100644
--- a/Source/web/StorageAreaProxy.cpp
+++ b/Source/web/StorageAreaProxy.cpp
@@ -154,7 +154,7 @@ void StorageAreaProxy::dispatchLocalStorageEvent(const String& key, const String
     // FIXME: This looks suspicious. Why doesn't this use allPages instead?
     const HashSet<Page*>& pages = PageGroup::sharedGroup()->pages();
     for (HashSet<Page*>::const_iterator it = pages.begin(); it != pages.end(); ++it) {
-        for (Frame* frame = (*it)->mainFrame(); frame; frame = frame->tree()->traverseNext()) {
+        for (Frame* frame = (*it)->mainFrame(); frame; frame = frame->tree().traverseNext()) {
             Storage* storage = frame->domWindow()->optionalLocalStorage();
             if (storage && frame->document()->securityOrigin()->equal(securityOrigin) && !isEventSource(storage, sourceAreaInstance))
                 frame->document()->enqueueWindowEvent(StorageEvent::create(EventTypeNames::storage, key, oldValue, newValue, pageURL, storage));
@@ -184,7 +184,7 @@ void StorageAreaProxy::dispatchSessionStorageEvent(const String& key, const Stri
     if (!page)
         return;
 
-    for (Frame* frame = page->mainFrame(); frame; frame = frame->tree()->traverseNext()) {
+    for (Frame* frame = page->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         Storage* storage = frame->domWindow()->optionalSessionStorage();
         if (storage && frame->document()->securityOrigin()->equal(securityOrigin) && !isEventSource(storage, sourceAreaInstance))
             frame->document()->enqueueWindowEvent(StorageEvent::create(EventTypeNames::storage, key, oldValue, newValue, pageURL, storage));