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

cc/animation/scroll_offset_animations.h

Index: cc/animation/scroll_offset_animations.h
diff --git a/cc/animation/scroll_offset_animations.h b/cc/animation/scroll_offset_animations.h
index 26b07d52c273341b887a402346018ef9d2288cdb..421a31380cdf3bba8dc69de7cf14f674da56228e 100644
--- a/cc/animation/scroll_offset_animations.h
+++ b/cc/animation/scroll_offset_animations.h
@@ -5,6 +5,8 @@
 #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"
 
@@ -14,16 +16,18 @@ namespace cc {
 // 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 };
-
-  ScrollOffsetAnimationUpdate(Type, ElementId);
+  ScrollOffsetAnimationUpdate();
+  explicit ScrollOffsetAnimationUpdate(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.
@@ -34,7 +38,9 @@ class CC_EXPORT ScrollOffsetAnimations {
   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
@@ -42,8 +48,11 @@ class CC_EXPORT ScrollOffsetAnimations {
   void PushPropertiesTo(ScrollOffsetAnimationsImpl*);
 
  private:
-  using UpdatesQueue = std::vector<ScrollOffsetAnimationUpdate>;
-  UpdatesQueue queue_;
+  ScrollOffsetAnimationUpdate GetUpdateForElementId(ElementId) const;
+
+  using ElementToUpdateMap =
+      std::unordered_map<ElementId, ScrollOffsetAnimationUpdate>;
+  ElementToUpdateMap element_to_update_map_;
 
   AnimationHost* animation_host_;
 };