Have Frame::tree() return a reference

Source/core/page/PageGroup.cpp

Index: Source/core/page/PageGroup.cpp
diff --git a/Source/core/page/PageGroup.cpp b/Source/core/page/PageGroup.cpp
index d623f59c48e45a83aaf2346eeee6adda2b03d35e..c1632647fad1a1caaea4f9b96e62bf8b3b54c02e 100644
--- a/Source/core/page/PageGroup.cpp
+++ b/Source/core/page/PageGroup.cpp
@@ -85,7 +85,7 @@ void PageGroup::invalidatedInjectedStyleSheetCacheInAllFrames()
     // Clear our cached sheets and have them just reparse.
     HashSet<Page*>::const_iterator end = m_pages.end();
     for (HashSet<Page*>::const_iterator it = m_pages.begin(); it != end; ++it) {
-        for (Frame* frame = (*it)->mainFrame(); frame; frame = frame->tree()->traverseNext())
+        for (Frame* frame = (*it)->mainFrame(); frame; frame = frame->tree().traverseNext())
             frame->document()->styleEngine()->invalidateInjectedStyleSheetCache();
     }
 }