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

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 7f282487ab430da4f261687b5ac494deff270710..e9b0e2c21edae4d3ecbfb4977c1d8bc501f806fe 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -279,6 +279,8 @@ class CC_EXPORT Layer : public base::RefCounted<Layer>,
   bool user_scrollable_vertical() const { return user_scrollable_vertical_; }
 
   void AddMainThreadScrollingReasons(uint32_t main_thread_scrolling_reasons);
+  void ClearMainThreadScrollingReasons(
+      uint32_t main_thread_scrolling_reasons_to_clear);
   void ClearMainThreadScrollingReasons();
   uint32_t main_thread_scrolling_reasons() const {
     return main_thread_scrolling_reasons_;