Take MT jank into account when animating the scroll offset on CC

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 f72a2d80fa59531bef442f2988cc35bd91535451..2956dafead9a77e9b387dcf2024d6bddfe348f05 100644
--- a/cc/animation/scroll_offset_animations_impl.cc
+++ b/cc/animation/scroll_offset_animations_impl.cc
@@ -36,13 +36,14 @@ ScrollOffsetAnimationsImpl::~ScrollOffsetAnimationsImpl() {
 void ScrollOffsetAnimationsImpl::ScrollAnimationCreate(
     ElementId element_id,
     const gfx::ScrollOffset& target_offset,
-    const gfx::ScrollOffset& current_offset) {
+    const gfx::ScrollOffset& current_offset,
+    base::TimeDelta delayed_by) {
   std::unique_ptr<ScrollOffsetAnimationCurve> curve =
       ScrollOffsetAnimationCurve::Create(
           target_offset, CubicBezierTimingFunction::CreatePreset(
                              CubicBezierTimingFunction::EaseType::EASE_IN_OUT),
           ScrollOffsetAnimationCurve::DurationBehavior::INVERSE_DELTA);
-  curve->SetInitialValue(current_offset);
+  curve->SetInitialValue(current_offset, delayed_by);
 
   std::unique_ptr<Animation> animation = Animation::Create(
       std::move(curve), AnimationIdProvider::NextAnimationId(),
@@ -61,7 +62,8 @@ bool ScrollOffsetAnimationsImpl::ScrollAnimationUpdateTarget(
     ElementId element_id,
     const gfx::Vector2dF& scroll_delta,
     const gfx::ScrollOffset& max_scroll_offset,
-    base::TimeTicks frame_monotonic_time) {
+    base::TimeTicks frame_monotonic_time,
+    base::TimeDelta delayed_by) {
   DCHECK(scroll_offset_animation_player_);
   if (!scroll_offset_animation_player_->element_animations())
     return false;
@@ -93,6 +95,10 @@ bool ScrollOffsetAnimationsImpl::ScrollAnimationUpdateTarget(
           ? base::TimeDelta()
           : animation->TrimTimeToCurrentIteration(frame_monotonic_time);
 
+  // Re-target taking the delay into account.
+  if (delayed_by > base::TimeDelta())

[tdresser, 2016/08/25 15:55:54]

!delayed_by.is_zero()

[ymalik, 2016/08/29 14:52:05]

Covered this on chat, but for posterity:
delayed_by is a TimeDelta and can be negative, so this prevents against  
non-sane values.

+    trimmed = delayed_by < trimmed ? trimmed - delayed_by : base::TimeDelta();

[tdresser, 2016/08/25 15:55:54]

I'm not completely clear on what's going on here.

We're trimming time off the front?

I'm not sure how overflow works with TimeDeltas, but we should be sure this
makes sense.

[tdresser, 2016/08/29 14:31:33]

Subtraction is just saturated. This seems reasonable.

[ymalik, 2016/08/29 14:52:05]

Yes. We're just trying to use the delay as a signal that this should have really
been re-targeted delayed_by ago.

+
   curve->UpdateTarget(trimmed.InSecondsF(), new_target);
 
   return true;