Have Frame::tree() return a reference

Source/core/dom/TouchController.cpp

Index: Source/core/dom/TouchController.cpp
diff --git a/Source/core/dom/TouchController.cpp b/Source/core/dom/TouchController.cpp
index ef6b65fe0048cf9e8314c377d249230bf958923d..a24bc81c1662fd4e029d15f0b60c406a6b1d6419 100644
--- a/Source/core/dom/TouchController.cpp
+++ b/Source/core/dom/TouchController.cpp
@@ -97,7 +97,7 @@ void TouchController::didRemoveTouchEventHandler(Document* document, Node* handl
         scrollingCoordinator->touchEventTargetRectsDidChange(document);
     if (m_touchEventTargets->size())
         return;
-    for (const Frame* frame = page->mainFrame(); frame; frame = frame->tree()->traverseNext()) {
+    for (const Frame* frame = page->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         if (frame->document() && TouchController::from(frame->document())->hasTouchEventHandlers())
             return;
     }