Communicate MT changes to impl-only scroll offset animations

cc/animation/animation_host.h

Index: cc/animation/animation_host.h
diff --git a/cc/animation/animation_host.h b/cc/animation/animation_host.h
index 5839d599753d50fd138162b541bd1618cb1b4c3a..a8fa6017c6033522e3ee4a1810999696646c2170 100644
--- a/cc/animation/animation_host.h
+++ b/cc/animation/animation_host.h
@@ -13,7 +13,6 @@
 #include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 #include "cc/animation/animation.h"
-#include "cc/animation/scroll_offset_animations_impl.h"
 #include "cc/base/cc_export.h"
 #include "cc/trees/mutator_host_client.h"
 #include "ui/gfx/geometry/box_f.h"
@@ -30,6 +29,8 @@ class AnimationPlayer;
 class AnimationTimeline;
 class ElementAnimations;
 class LayerTreeHost;
+class ScrollOffsetAnimations;
+class ScrollOffsetAnimationsImpl;
 
 enum class ThreadInstance { MAIN, IMPL };
 
@@ -149,6 +150,9 @@ class CC_EXPORT AnimationHost {
 
   void ScrollAnimationAbort(bool needs_completion);
 
+  // This should only be called from the main thread.
+  ScrollOffsetAnimations& scroll_offset_animations() const;
+
   // Registers the given element animations as active. An active element
   // animations is one that has a running animation that needs to be ticked.
   void DidActivateElementAnimations(ElementAnimations* element_animations);
@@ -189,6 +193,7 @@ class CC_EXPORT AnimationHost {
 
   MutatorHostClient* mutator_host_client_;
 
+  std::unique_ptr<ScrollOffsetAnimations> scroll_offset_animations_;
   std::unique_ptr<ScrollOffsetAnimationsImpl> scroll_offset_animations_impl_;
 
   const ThreadInstance thread_instance_;