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

cc/input/main_thread_scrolling_reason.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 #define CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 
 namespace cc {
 
 // Ensure this stays in sync with MainThreadScrollingReason in histograms.xml.
 // When adding a new MainThreadScrollingReason, make sure the corresponding
 // [MainThread/Compositor]CanSetScrollReasons function is also updated.
 struct MainThreadScrollingReason {
   // Non-transient scrolling reasons.
   enum : uint32_t { kNotScrollingOnMain = 0 };
   enum : uint32_t { kHasBackgroundAttachmentFixedObjects = 1 << 0 };
   enum : uint32_t { kHasNonLayerViewportConstrainedObjects = 1 << 1 };
   enum : uint32_t { kThreadedScrollingDisabled = 1 << 2 };
   enum : uint32_t { kScrollbarScrolling = 1 << 3 };
   enum : uint32_t { kPageOverlay = 1 << 4 };
+  enum : uint32_t { kAnimatingScollOnMainThread = 1 << 13 };
 
   // Transient scrolling reasons. These are computed for each scroll begin.
   enum : uint32_t { kNonFastScrollableRegion = 1 << 5 };
   enum : uint32_t { kEventHandlers = 1 << 6 };
   enum : uint32_t { kFailedHitTest = 1 << 7 };
   enum : uint32_t { kNoScrollingLayer = 1 << 8 };
   enum : uint32_t { kNotScrollable = 1 << 9 };
   enum : uint32_t { kContinuingMainThreadScroll = 1 << 10 };
   enum : uint32_t { kNonInvertibleTransform = 1 << 11 };
   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.
   static bool MainThreadCanSetScrollReasons(uint32_t reasons) {
     uint32_t reasons_set_by_main_thread =
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
-        kScrollbarScrolling | kPageOverlay;
+        kScrollbarScrolling | kPageOverlay | kAnimatingScollOnMainThread;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }
 
   // Returns true if the given MainThreadScrollingReason can be set by the
   // compositor.
   static bool CompositorCanSetScrollReasons(uint32_t reasons) {
     uint32_t reasons_set_by_compositor =
         kNonFastScrollableRegion | kEventHandlers | kFailedHitTest |
         kNoScrollingLayer | kNotScrollable | kContinuingMainThreadScroll |
         kNonInvertibleTransform | kPageBasedScrolling;
     return (reasons & reasons_set_by_compositor) == reasons;
   }
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_