Fix smooth scroll jump when switching scroll handling between MT and CC

third_party/WebKit/public/platform/WebLayer.h

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 183 matching lines @@
 
     // To set a WebLayer as scrollable we must specify the corresponding clip layer.
     virtual void setScrollClipLayer(WebLayer*) = 0;
     virtual bool scrollable() const = 0;
     virtual void setUserScrollable(bool horizontal, bool vertical) = 0;
     virtual bool userScrollableHorizontal() const = 0;
     virtual bool userScrollableVertical() const = 0;
 
     // Indicates that this layer will always scroll on the main thread for the provided reason.
     virtual void addMainThreadScrollingReasons(uint32_t) = 0;
-    // Indicates that the layer could scroll on the compositor thread.
-    virtual void clearMainThreadScrollingReasons() = 0;
+    virtual void clearMainThreadScrollingReasons(uint32_t mainThreadScrollingReasonsToClear) = 0;
+    virtual uint32_t mainThreadScrollingReasons() = 0;
     virtual bool shouldScrollOnMainThread() const = 0;
 
     virtual void setNonFastScrollableRegion(const WebVector<WebRect>&) = 0;
     virtual WebVector<WebRect> nonFastScrollableRegion() const = 0;
 
     virtual void setTouchEventHandlerRegion(const WebVector<WebRect>&) = 0;
     virtual WebVector<WebRect> touchEventHandlerRegion() const = 0;
 
     // Setter and getter for Frame Timing rects.
     // See http://w3c.github.io/frame-timing/ for definition of terms.
@@ skipping 28 matching lines @@
     virtual void setElementId(uint64_t) = 0;
     virtual uint64_t elementId() const = 0;
 
     virtual void setCompositorMutableProperties(uint32_t) = 0;
     virtual uint32_t compositorMutableProperties() const = 0;
 };
 
 } // namespace blink
 
 #endif // WebLayer_h