Reduce composited smooth scroll latency by a frame (also fixes latency UMAs).

cc/animation/animation_host.h

 // Copyright 2015 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_ANIMATION_HOST_H_
 #define CC_ANIMATION_ANIMATION_HOST_H_
 
 #include <memory>
 #include <unordered_map>
 #include <vector>
@@ skipping 137 matching lines @@
   bool MaximumTargetScale(ElementId element_id,
                           ElementListType list_type,
                           float* max_scale) const override;
   bool AnimationStartScale(ElementId element_id,
                            ElementListType list_type,
                            float* start_scale) const override;
 
   bool HasAnyAnimation(ElementId element_id) const override;
   bool HasTickingAnimationForTesting(ElementId element_id) const override;
 
-  void ImplOnlyScrollAnimationCreate(ElementId element_id,
-                                     const gfx::ScrollOffset& target_offset,
-                                     const gfx::ScrollOffset& current_offset,
-                                     base::TimeDelta delayed_by) override;
+  void ImplOnlyScrollAnimationCreate(
+      ElementId element_id,
+      const gfx::ScrollOffset& target_offset,
+      const gfx::ScrollOffset& current_offset,
+      base::TimeDelta delayed_by,
+      base::TimeDelta animation_start_offset) override;
   bool ImplOnlyScrollAnimationUpdateTarget(
       ElementId element_id,
       const gfx::Vector2dF& scroll_delta,
       const gfx::ScrollOffset& max_scroll_offset,
       base::TimeTicks frame_monotonic_time,
       base::TimeDelta delayed_by) override;
 
   void ScrollAnimationAbort() override;
 
   // This should only be called from the main thread.
@@ skipping 36 matching lines @@
 
   bool supports_scroll_animations_;
   bool needs_push_properties_;
 
   DISALLOW_COPY_AND_ASSIGN(AnimationHost);
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_ANIMATION_HOST_H_