Have Frame::tree() return a reference

Source/core/inspector/PageRuntimeAgent.cpp

Index: Source/core/inspector/PageRuntimeAgent.cpp
diff --git a/Source/core/inspector/PageRuntimeAgent.cpp b/Source/core/inspector/PageRuntimeAgent.cpp
index aa8bac6c0eb37d91ce0ebe868d05c06a05b44ef9..040c026fbbe997d1615aa17b956a05cfb83799ea 100644
--- a/Source/core/inspector/PageRuntimeAgent.cpp
+++ b/Source/core/inspector/PageRuntimeAgent.cpp
@@ -161,7 +161,7 @@ void PageRuntimeAgent::unmuteConsole()
 void PageRuntimeAgent::reportExecutionContextCreation()
 {
     Vector<std::pair<ScriptState*, SecurityOrigin*> > isolatedContexts;
-    for (Frame* frame = m_inspectedPage->mainFrame(); frame; frame = frame->tree()->traverseNext()) {
+    for (Frame* frame = m_inspectedPage->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         if (!frame->script()->canExecuteScripts(NotAboutToExecuteScript))
             continue;
         String frameId = m_pageAgent->frameId(frame);