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

cc/animation/scroll_offset_animations_impl.h

 // 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_IMPL_H_
 #define CC_ANIMATION_SCROLL_OFFSET_ANIMATIONS_IMPL_H_
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
@@ skipping 13 matching lines @@
 // We have just one player for impl-only scroll offset animations. I.e. only
 // one element can have an impl-only scroll offset animation at any given time.
 // Note that this class only exists on the compositor thread.
 class CC_ANIMATION_EXPORT ScrollOffsetAnimationsImpl
     : public AnimationDelegate {
  public:
   explicit ScrollOffsetAnimationsImpl(AnimationHost* animation_host);
 
   ~ScrollOffsetAnimationsImpl() override;
 
+  // |delayed_by| shrinks the duration of the
+  // animation. |animation_start_offset| causes us to start the animation
+  // partway through.
   void ScrollAnimationCreate(ElementId element_id,
                              const gfx::ScrollOffset& target_offset,
                              const gfx::ScrollOffset& current_offset,
-                             base::TimeDelta delayed_by);
+                             base::TimeDelta delayed_by,
+                             base::TimeDelta animation_start_offset);
 
   bool ScrollAnimationUpdateTarget(ElementId element_id,
                                    const gfx::Vector2dF& scroll_delta,
                                    const gfx::ScrollOffset& max_scroll_offset,
                                    base::TimeTicks frame_monotonic_time,
                                    base::TimeDelta delayed_by);
 
   // Aborts the currently running scroll offset animation on an element and
   // starts a new one offsetted by adjustment.
   void ScrollAnimationApplyAdjustment(ElementId element_id,
@@ skipping 27 matching lines @@
   // I.e. only one element can have an impl-only scroll offset animation at
   // any given time.
   scoped_refptr<AnimationPlayer> scroll_offset_animation_player_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollOffsetAnimationsImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_SCROLL_OFFSET_ANIMATIONS_IMPL_H_