Disable clipping on root scroller's ancestors.

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

 // 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.
 
 #ifndef TopDocumentRootScrollerController_h
 #define TopDocumentRootScrollerController_h
 
 #include "core/CoreExport.h"
 #include "core/page/scrolling/RootScrollerController.h"
 #include "platform/heap/Handle.h"
@@ skipping 35 matching lines @@
     // ViewportScrollCallback managed by this class.
     // TODO(bokan): Temporarily needed to allow ScrollCustomization to
     // differentiate between real custom callback and the built-in viewport
     // apply scroll. crbug.com/623079.
     bool isViewportScrollCallback(
         const ScrollStateCallback*) const override;
 
     // Returns the GraphicsLayer for the global root scroller.
     GraphicsLayer* rootScrollerLayer() override;
 
+    // Returns the Element that's the global root scroller.
+    Element* globalRootScroller() const;
+
 protected:
     TopDocumentRootScrollerController(Document&);
 
     // Called when the root scroller of descendant frames changes.
     void globalRootScrollerMayHaveChanged() override;
 
 private:
     // Calculates the Element that should be the globalRootScroller. On a
     // simple page, this will simply the root frame's effectiveRootScroller but
     // if the root scroller is set to an iframe, this will then descend into
@@ skipping 10 matching lines @@
     Member<ViewportScrollCallback> m_viewportApplyScroll;
 
     // The page level root scroller. i.e. The actual element for which scrolling
     // should move top controls and produce overscroll glow.
     WeakMember<Element> m_globalRootScroller;
 };
 
 } // namespace blink
 
 #endif // RootScrollerController_h