Communicate MT changes to impl-only scroll offset animations

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 ffebdbcd5e76bdae3f1666dfd8f9384d4235b25d..ebe24dfb89552bee4472de8380dc9094127841ef 100644
--- a/cc/animation/scroll_offset_animations_impl.cc
+++ b/cc/animation/scroll_offset_animations_impl.cc
@@ -90,6 +90,45 @@ bool ScrollOffsetAnimationsImpl::ScrollAnimationUpdateTarget(
   return true;
 }
 
+void ScrollOffsetAnimationsImpl::ScrollAnimationApplyAdjustment(
+    ElementId element_id,
+    const gfx::Vector2dF& adjustment,
+    base::TimeTicks frame_monotonic_time) {
+  DCHECK(scroll_offset_animation_player_);
+  if (element_id != scroll_offset_animation_player_->element_id())
+    return;
+
+  if (!scroll_offset_animation_player_->element_animations())
+    return;
+
+  Animation* animation =
+      scroll_offset_animation_player_->element_animations()->GetAnimation(
+          TargetProperty::SCROLL_OFFSET);
+  if (!animation)
+    return;
+
+  // Abort the old animation.
+  ScrollAnimationAbort(/* needs_completion */ false);
+
+  // Start a new one with the adjusment.
+  std::unique_ptr<ScrollOffsetAnimationCurve> new_curve =
+      animation->curve()
+          ->ToScrollOffsetAnimationCurve()
+          ->CloneToScrollOffsetAnimationCurve();
+  new_curve->ApplyAdjustment(
+      animation->TrimTimeToCurrentIteration(frame_monotonic_time), adjustment);
+
+  std::unique_ptr<Animation> new_animation = Animation::Create(
+      std::move(new_curve), AnimationIdProvider::NextAnimationId(),
+      AnimationIdProvider::NextGroupId(), TargetProperty::SCROLL_OFFSET);
+  new_animation->set_is_impl_only(true);
+  new_animation->set_affects_active_elements(false);
+
+  ReattachScrollOffsetPlayerIfNeeded(element_id);

[loyso (OOO), 2016/05/06 01:04:38]

element_id == scroll_offset_animation_player_->element_id here. Do we need to
reattach? Also, is it a valid case when we keep spaming impl thread with these
udates? Can we replace element_id !=
scroll_offset_animation_player_->element_id() with a DCHECK?

[ymalik, 2016/05/06 05:41:49]

You're right, no need to reattach.

We can't replay it with a DCHECK in case the adjustment is applied to a layer
that we're not currently scrolling.

+
+  scroll_offset_animation_player_->AddAnimation(std::move(new_animation));
+}
+
 void ScrollOffsetAnimationsImpl::ScrollAnimationAbort(bool needs_completion) {
   DCHECK(scroll_offset_animation_player_);
   scroll_offset_animation_player_->AbortAnimations(