Communicate MT changes to impl-only scroll offset animations

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
new file mode 100644
index 0000000000000000000000000000000000000000..26b07d52c273341b887a402346018ef9d2288cdb
--- /dev/null
+++ b/cc/animation/scroll_offset_animations.h
@@ -0,0 +1,53 @@
+// 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 "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 };
+
+  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_;
+};
+
+// 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);
+  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_;
+
+  AnimationHost* animation_host_;
+};
+
+}  // namespace cc
+
+#endif  // CC_ANIMATION_SCROLL_OFFSET_ANIMATIONS_H_