Make RootScroller set the outer viewport scroll layer in the compositor

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

Index: third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h b/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
index b42428a55f420b53fde6b5d51ce6e44532789db4..57808daa805b68316a58a208c4d2d0537a01475b 100644
--- a/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
+++ b/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
@@ -81,6 +81,10 @@ public:
     // 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.
@@ -104,6 +108,11 @@ private:
 
     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