Make all gesture scrolls use customization path internally

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 2d5261c6694b74edc0f9c37e7be8cb1b699cd363..b42428a55f420b53fde6b5d51ce6e44532789db4 100644
--- a/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
+++ b/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
@@ -50,9 +50,11 @@ public:
     }
 
     // Creates an apply scroll callback that handles viewport actions like
-    // TopControls movement and Overscroll.
+    // TopControls movement and Overscroll. The TopControls and
+    // OverscrollController are given to the ViewportScrollCallback but are not
+    // owned or kept alive by it.
     static ViewportScrollCallback* createViewportApplyScroll(
-        TopControls&, OverscrollController&);
+        TopControls*, OverscrollController*);
 
     DECLARE_TRACE();
 
@@ -79,6 +81,14 @@ public:
     // replaced by the defualt root scroller.
     void didUpdateLayout();
 
+    // 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*);