Remove scroll_clip_layer dependency from viewport layer tracking

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/FrameView.h"
 #include "core/frame/PageScaleConstraintsSet.h"
@@ skipping 184 matching lines @@
 
   GraphicsLayer* graphics_layer = 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 graphics_layer;
 }
 
+GraphicsLayer* TopDocumentRootScrollerController::RootContainerLayer() const {
+  ScrollableArea* area =
+      RootScrollerUtil::ScrollableAreaForRootScroller(global_root_scroller_);
+
+  return area ? area->LayerForContainer() : nullptr;
+}
+
 PaintLayer* TopDocumentRootScrollerController::RootScrollerPaintLayer() const {
   return RootScrollerUtil::PaintLayerForRootScroller(global_root_scroller_);
 }
 
 Element* TopDocumentRootScrollerController::GlobalRootScroller() const {
   return global_root_scroller_.Get();
 }
 
 }  // namespace blink