Use const ref for LocalFrame::LocalFrameRoot and FrameTree::Top

third_party/WebKit/Source/core/frame/DOMWindow.cpp

Index: third_party/WebKit/Source/core/frame/DOMWindow.cpp
diff --git a/third_party/WebKit/Source/core/frame/DOMWindow.cpp b/third_party/WebKit/Source/core/frame/DOMWindow.cpp
index 308c99c1fa0091a12aca8bafaba1465489de8a8d..84ccd6579348700645e4fb40ad77627de5c06d40 100644
--- a/third_party/WebKit/Source/core/frame/DOMWindow.cpp
+++ b/third_party/WebKit/Source/core/frame/DOMWindow.cpp
@@ -106,7 +106,7 @@ DOMWindow* DOMWindow::top() const {
   if (!GetFrame())
     return nullptr;
 
-  return GetFrame()->Tree().Top()->DomWindow();
+  return GetFrame()->Tree().Top().DomWindow();
 }
 
 DOMWindow* DOMWindow::AnonymousIndexedGetter(uint32_t index) const {
@@ -228,7 +228,7 @@ void DOMWindow::postMessage(PassRefPtr<SerializedScriptValue> message,
                  source_document->Url())) {
     UseCounter::Count(GetFrame(), UseCounter::kPostMessageFromInsecureToSecure);
     if (MixedContentChecker::IsMixedContent(
-            GetFrame()->Tree().Top()->GetSecurityContext()->GetSecurityOrigin(),
+            GetFrame()->Tree().Top().GetSecurityContext()->GetSecurityOrigin(),
             source_document->Url())) {
       UseCounter::Count(GetFrame(),
                         UseCounter::kPostMessageFromInsecureToSecureToplevel);