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

cc/input/main_thread_scrolling_reason.h

Index: cc/input/main_thread_scrolling_reason.h
diff --git a/cc/input/main_thread_scrolling_reason.h b/cc/input/main_thread_scrolling_reason.h
index 7e921fc021227b438361317860aaf3b5d415d2ac..1f384ebfcee48d60cee17f942d8d3128b5c1e632 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -18,6 +18,7 @@ struct MainThreadScrollingReason {
   enum : uint32_t { kThreadedScrollingDisabled = 1 << 2 };
   enum : uint32_t { kScrollbarScrolling = 1 << 3 };
   enum : uint32_t { kPageOverlay = 1 << 4 };
+  enum : uint32_t { kAnimatingScrollOnMainThread = 1 << 13 };
 
   // Transient scrolling reasons. These are computed for each scroll begin.
   enum : uint32_t { kNonFastScrollableRegion = 1 << 5 };
@@ -30,7 +31,7 @@ struct MainThreadScrollingReason {
   enum : uint32_t { kPageBasedScrolling = 1 << 12 };
 
   // The number of flags in this struct (excluding itself).
-  enum : uint32_t { kMainThreadScrollingReasonCount = 14 };
+  enum : uint32_t { kMainThreadScrollingReasonCount = 15 };
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
   // thread.
@@ -38,7 +39,7 @@ struct MainThreadScrollingReason {
     uint32_t reasons_set_by_main_thread =
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
-        kScrollbarScrolling | kPageOverlay;
+        kScrollbarScrolling | kPageOverlay | kAnimatingScrollOnMainThread;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }