Make RootScroller set the outer viewport scroll layer in the compositor

third_party/WebKit/Source/core/page/scrolling/RootScrollerController.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 RootScrollerController_h
 #define RootScrollerController_h
 
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
 
@@ skipping 63 matching lines @@
     // This returns the Element that's actually being used to control viewport
     // actions right now. This is different from get() if a root scroller hasn't
     // been set, or if the set root scroller isn't currently a valid scroller.
     Element* effectiveRootScroller() const;
 
     // This class needs to be informed of changes in layout so that it can
     // determine if the current root scroller is still valid or if it must be
     // replaced by the defualt root scroller.
     void didUpdateLayout();
 
+    // This class needs to be informed of changes to compositing so that it can
+    // update the compositor when the effective root scroller changes.
+    void didUpdateCompositing();
+
     // TODO(bokan): Temporarily exposed to allow ScrollCustomization to
     // differentiate between real custom callback and the built-in viewport
     // apply scroll.
     const ViewportScrollCallback* viewportScrollCallback()
     {
         return m_viewportApplyScroll;
     }
 
 private:
     RootScrollerController(Document&, ViewportScrollCallback*);
 
     Element* defaultEffectiveRootScroller();
 
     // Ensures the effective root scroller is currently valid and replaces it
     // with the default if not.
     void updateEffectiveRootScroller();
     void moveViewportApplyScroll(Element* target);
 
     WeakMember<Document> m_document;
     Member<ViewportScrollCallback> m_viewportApplyScroll;
 
     WeakMember<Element> m_rootScroller;
     WeakMember<Element> m_effectiveRootScroller;
+
+    // Keeps track of whether we need to notify the compositor about a changed
+    // effective root scroller. This can only be done after a compositing update
+    // once the layer tree has been updated.
+    bool m_changedSinceLastCompositingUpdate;

[aelias_OOO_until_Jul13, 2016/06/29 22:44:08]

I doubt that this dirty bit is really needed.  Can you just compare every time
to the old value (e.g. the one held by LayerTreeHost)?

[bokan, 2016/07/07 21:20:45]

Done.

 };
 
 } // namespace blink
 
 #endif // RootScrollerController_h