Disable clipping on root scroller's ancestors.

third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/page/scrolling/TopDocumentRootScrollerController.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/Element.h"
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
@@ skipping 76 matching lines @@
     // Ideally, scroll customization would pass the current element to scroll to
     // the apply scroll callback but this doesn't happen today so we set it
     // through a back door here. This is also needed by the
     // ViewportScrollCallback to swap the target into the layout viewport
     // in RootFrameViewport.
     m_viewportApplyScroll->setScroller(targetScroller);
 }
 
 void TopDocumentRootScrollerController::didUpdateCompositing()
 {
-    FrameHost* frameHost = m_document->frameHost();
+    RootScrollerController::didUpdateCompositing();
 
     // Let the compositor-side counterpart know about this change.
-    if (frameHost)
+    if (FrameHost* frameHost = m_document->frameHost())
         frameHost->chromeClient().registerViewportLayers();
 }
 
 void TopDocumentRootScrollerController::didAttachDocument()
 {
     FrameHost* frameHost = m_document->frameHost();
     FrameView* frameView = m_document->view();
 
     if (!frameHost || !frameView)
         return;
@@ skipping 31 matching lines @@
     GraphicsLayer* graphicsLayer = area->layerForScrolling();
 
     // TODO(bokan): We should assert graphicsLayer here and
     // RootScrollerController should do whatever needs to happen to ensure
     // the root scroller gets composited.
 
     return graphicsLayer;
 }
 
 } // namespace blink