Send takeover msg from MT to CC using the animation path.

cc/animation/scroll_offset_animations.h

 // Copyright (c) 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_ANIMATION_SCROLL_OFFSET_ANIMATIONS_H_
 #define CC_ANIMATION_SCROLL_OFFSET_ANIMATIONS_H_
 
+#include <unordered_map>
+
 #include "cc/animation/scroll_offset_animations_impl.h"
 #include "cc/trees/mutator_host_client.h"
 
 namespace cc {
 
 // A ScrollOffsetAnimationUpdate represents a change on the main thread
 // that may impact an impl-only scroll offset animation.
 // Note that this class only exists on the main thread.
 struct CC_EXPORT ScrollOffsetAnimationUpdate {
-  enum Type { SCROLL_OFFSET_CHANGED };
+  explicit ScrollOffsetAnimationUpdate(ElementId);
 
-  ScrollOffsetAnimationUpdate(Type, ElementId);
-
-  Type type_;
   // The element to which this update applies.
   ElementId element_id_;
 
   // The amount by which the scroll offset is changed on the main thread.
   gfx::Vector2dF adjustment_;
+
+  // If set to true, abort the currently running impl only scroll offset
+  // animation running on cc and finish it on the main thread.
+  bool takeover_;
 };
 
 // ScrollOffsetAnimations contains a list of ScrollOffsetAnimationUpdates.
 // PushPropertiesTo is called during commit time and the necessary update is
 // made to the impl-only animation.
 class CC_EXPORT ScrollOffsetAnimations {
  public:
   explicit ScrollOffsetAnimations(AnimationHost* animation_host);
   ~ScrollOffsetAnimations();
 
-  void AddUpdate(ScrollOffsetAnimationUpdate);
+  void AddAdjustmentUpdate(ElementId, gfx::Vector2dF adjustment);
+  void AddTakeoverUpdate(ElementId);
+
   bool HasUpdatesForTesting() const;
 
   // Goes through the updates in the order in which they're added and calls the
   // appropriate handler for each update.
   void PushPropertiesTo(ScrollOffsetAnimationsImpl*);
 
  private:
-  using UpdatesQueue = std::vector<ScrollOffsetAnimationUpdate>;
-  UpdatesQueue queue_;
+  ScrollOffsetAnimationUpdate GetUpdateForElementId(ElementId) const;
+
+  using ElementToUpdate =

[skobes, 2016/05/24 23:50:50]

I'd call this UpdateMap or maybe ElementToUpdateMap.

[ymalik, 2016/05/25 16:09:05]

Done.

+      std::unordered_map<ElementId, ScrollOffsetAnimationUpdate>;
+  ElementToUpdate element_to_update_map_;
 
   AnimationHost* animation_host_;
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_SCROLL_OFFSET_ANIMATIONS_H_