Have Frame::tree() return a reference

Source/core/page/Chrome.cpp

Index: Source/core/page/Chrome.cpp
diff --git a/Source/core/page/Chrome.cpp b/Source/core/page/Chrome.cpp
index 5d5c9ad9f2979e52c436c581038012854f74de45..e7a92ea06943472284a48d602de65cfea701bb56 100644
--- a/Source/core/page/Chrome.cpp
+++ b/Source/core/page/Chrome.cpp
@@ -158,7 +158,7 @@ bool Chrome::canRunModal() const
 
 static bool canRunModalIfDuringPageDismissal(Page* page, ChromeClient::DialogType dialog, const String& message)
 {
-    for (Frame* frame = page->mainFrame(); frame; frame = frame->tree()->traverseNext()) {
+    for (Frame* frame = page->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         Document::PageDismissalType dismissal = frame->document()->pageDismissalEventBeingDispatched();
         if (dismissal != Document::NoDismissal)
             return page->chrome().client().shouldRunModalDialogDuringPageDismissal(dialog, message, dismissal);