Have Frame::tree() return a reference

Source/core/rendering/RenderLayerCompositor.cpp

Index: Source/core/rendering/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/RenderLayerCompositor.cpp b/Source/core/rendering/RenderLayerCompositor.cpp
index 6b60b9c0839e0e2035d93e25ddffc7823c49186e..5f4a9177a92cd8eaeaa89117e1a21e0906907d81 100644
--- a/Source/core/rendering/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/RenderLayerCompositor.cpp
@@ -448,7 +448,7 @@ void RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
         m_secondaryBackingStoreBytes = 0;
 
         Frame& frame = m_renderView->frameView()->frame();
-        LOG(Compositing, "\nUpdate %d of %s.\n", m_rootLayerUpdateCount, isMainFrame() ? "main frame" : frame.tree()->uniqueName().string().utf8().data());
+        LOG(Compositing, "\nUpdate %d of %s.\n", m_rootLayerUpdateCount, isMainFrame() ? "main frame" : frame.tree().uniqueName().string().utf8().data());
     }
 #endif
 
@@ -2576,7 +2576,7 @@ void RenderLayerCompositor::notifyIFramesOfCompositingChange()
         return;
     Frame& frame = m_renderView->frameView()->frame();
 
-    for (Frame* child = frame.tree()->firstChild(); child; child = child->tree()->traverseNext(&frame)) {
+    for (Frame* child = frame.tree().firstChild(); child; child = child->tree().traverseNext(&frame)) {
         if (child->document() && child->document()->ownerElement())
             child->document()->ownerElement()->scheduleLayerUpdate();
     }