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

cc/animation/scroll_offset_animations_impl.cc

Index: cc/animation/scroll_offset_animations_impl.cc
diff --git a/cc/animation/scroll_offset_animations_impl.cc b/cc/animation/scroll_offset_animations_impl.cc
index 71a96b429f5938cb0c9f20ff6bf95cf88d09c0f9..512133cde53c580e25da5271fcf9bcef46f077b9 100644
--- a/cc/animation/scroll_offset_animations_impl.cc
+++ b/cc/animation/scroll_offset_animations_impl.cc
@@ -36,7 +36,8 @@ 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(
@@ -47,6 +48,7 @@ void ScrollOffsetAnimationsImpl::ScrollAnimationCreate(
   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_);