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

cc/trees/mutator_host.h

 // Copyright 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_TREES_MUTATOR_HOST_H_
 #define CC_TREES_MUTATOR_HOST_H_
 
 #include <memory>
 
 #include "base/memory/ptr_util.h"
@@ skipping 98 matching lines @@
                                    ElementListType list_type,
                                    float* start_scale) const = 0;
 
   virtual bool HasAnyAnimation(ElementId element_id) const = 0;
   virtual bool HasTickingAnimationForTesting(ElementId element_id) const = 0;
 
   virtual void ImplOnlyScrollAnimationCreate(
       ElementId element_id,
       const gfx::ScrollOffset& target_offset,
       const gfx::ScrollOffset& current_offset,
-      base::TimeDelta delayed_by) = 0;
+      base::TimeDelta delayed_by,
+      base::TimeDelta animation_start_offset) = 0;
   virtual 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) = 0;
 
   virtual void ScrollAnimationAbort() = 0;
 };
 
 class MutatorEvents {
  public:
   virtual ~MutatorEvents() {}
   virtual bool IsEmpty() const = 0;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_MUTATOR_HOST_H_