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

cc/animation/scroll_offset_animations_impl.cc

 // Copyright 2013 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.
 
 #include "cc/animation/scroll_offset_animations_impl.h"
 
 #include "cc/animation/animation_host.h"
 #include "cc/animation/animation_id_provider.h"
 #include "cc/animation/animation_player.h"
 #include "cc/animation/animation_timeline.h"
@@ skipping 18 matching lines @@
 
 ScrollOffsetAnimationsImpl::~ScrollOffsetAnimationsImpl() {
   scroll_offset_timeline_->DetachPlayer(scroll_offset_animation_player_.get());
   animation_host_->RemoveAnimationTimeline(scroll_offset_timeline_.get());
 }
 
 void ScrollOffsetAnimationsImpl::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) {
   std::unique_ptr<ScrollOffsetAnimationCurve> curve =
       ScrollOffsetAnimationCurve::Create(
           target_offset, CubicBezierTimingFunction::CreatePreset(
                              CubicBezierTimingFunction::EaseType::EASE_IN_OUT),
           ScrollOffsetAnimationCurve::DurationBehavior::INVERSE_DELTA);
   curve->SetInitialValue(current_offset, delayed_by);
 
   std::unique_ptr<Animation> animation = Animation::Create(
       std::move(curve), AnimationIdProvider::NextAnimationId(),
       AnimationIdProvider::NextGroupId(), TargetProperty::SCROLL_OFFSET);
+  animation->set_time_offset(animation_start_offset);
   animation->set_is_impl_only(true);
 
   DCHECK(scroll_offset_animation_player_);
   DCHECK(scroll_offset_animation_player_->animation_timeline());
 
   ReattachScrollOffsetPlayerIfNeeded(element_id);
 
   scroll_offset_animation_player_->AddAnimation(std::move(animation));
 }
 
@@ skipping 97 matching lines @@
     ElementId element_id) {
   if (scroll_offset_animation_player_->element_id() != element_id) {
     if (scroll_offset_animation_player_->element_id())
       scroll_offset_animation_player_->DetachElement();
     if (element_id)
       scroll_offset_animation_player_->AttachElement(element_id);
   }
 }
 
 }  // namespace cc