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

cc/animation/scroll_offset_animation_curve.cc

Index: cc/animation/scroll_offset_animation_curve.cc
diff --git a/cc/animation/scroll_offset_animation_curve.cc b/cc/animation/scroll_offset_animation_curve.cc
index b207994c092a327a70c753d99b7c93f4446bf4c2..56c3d7ad6c0196aabb64d33cbf470263f0073a02 100644
--- a/cc/animation/scroll_offset_animation_curve.cc
+++ b/cc/animation/scroll_offset_animation_curve.cc
@@ -41,7 +41,8 @@ static float MaximumDimension(const gfx::Vector2dF& delta) {
 }
 
 static base::TimeDelta SegmentDuration(const gfx::Vector2dF& delta,
-                                       DurationBehavior behavior) {
+                                       DurationBehavior behavior,
+                                       base::TimeDelta delayed_by) {
   double duration = kConstantDuration;
   switch (behavior) {
     case DurationBehavior::CONSTANT:
@@ -60,8 +61,14 @@ static base::TimeDelta SegmentDuration(const gfx::Vector2dF& delta,
     default:
       NOTREACHED();
   }
-  return base::TimeDelta::FromMicroseconds(duration / kDurationDivisor *
-                                           base::Time::kMicrosecondsPerSecond);
+
+  base::TimeDelta td = base::TimeDelta::FromMicroseconds(

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

td -> time_delta

https://google.github.io/styleguide/cppguide.html#General_Naming_Rules
'Give as descriptive a name as possible, within reason. Do not worry about
saving horizontal space as it is far more important to make your code
immediately understandable by a new reader. Do not use abbreviations that are
ambiguous or unfamiliar to readers outside your project, and do not abbreviate
by deleting letters within a word.'

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

Great, thanks!

+      duration / kDurationDivisor * base::Time::kMicrosecondsPerSecond);
+
+  td = td - delayed_by;
+  if (td >= base::TimeDelta())

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

operator> is comparing unsigned integers, so this is always true...

Does
operator-(https://cs.chromium.org/chromium/src/base/time/time.h?rcl=1472109603&l=185)
do what you think it does?

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

Whoops, nevermind, this looks fine.

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

Acknowledged.

+    return td;
+  return base::TimeDelta();
 }
 
 static std::unique_ptr<TimingFunction> EaseOutWithInitialVelocity(
@@ -98,11 +105,12 @@ ScrollOffsetAnimationCurve::ScrollOffsetAnimationCurve(
 ScrollOffsetAnimationCurve::~ScrollOffsetAnimationCurve() {}
 
 void ScrollOffsetAnimationCurve::SetInitialValue(
-    const gfx::ScrollOffset& initial_value) {
+    const gfx::ScrollOffset& initial_value,
+    base::TimeDelta delayed_by) {
   initial_value_ = initial_value;
   has_set_initial_value_ = true;
   total_animation_duration_ = SegmentDuration(
-      target_value_.DeltaFrom(initial_value_), duration_behavior_);
+      target_value_.DeltaFrom(initial_value_), duration_behavior_, delayed_by);
 }
 
 bool ScrollOffsetAnimationCurve::HasSetInitialValue() const {
@@ -120,6 +128,9 @@ gfx::ScrollOffset ScrollOffsetAnimationCurve::GetValue(
   base::TimeDelta duration = total_animation_duration_ - last_retarget_;
   t -= last_retarget_;
 
+  if (has_set_initial_value_ && duration == base::TimeDelta())

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

duration_.is_zero()

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

Done.

+    return target_value_;
+
   if (t <= base::TimeDelta())
     return initial_value_;
 
@@ -194,6 +205,13 @@ void ScrollOffsetAnimationCurve::UpdateTarget(
     target_value_ = new_target;
     return;
   }
+
+  if (t == 0) {

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

Should this be combined with the clause above?

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

I think this is more clear in communicating that this case is for when the
animation hasn't started yet, as opposed to the one above, where the new target
very small. WDYT?

[tdresser, 2016/08/29 15:28:28]

Acknowledged.

+    // Just update the target since the animation hasn't started yet.
+    target_value_ = new_target;
+    return;
+  }
+
   gfx::ScrollOffset current_position =
       GetValue(base::TimeDelta::FromSecondsD(t));
   gfx::Vector2dF old_delta = target_value_.DeltaFrom(initial_value_);
@@ -208,7 +226,8 @@ void ScrollOffsetAnimationCurve::UpdateTarget(
   // segment duration. This minimizes the "rubber-band" bouncing effect when
   // old_normalized_velocity is large and new_delta is small.
   double new_duration =
-      std::min(SegmentDuration(new_delta, duration_behavior_).InSecondsF(),
+      std::min(SegmentDuration(new_delta, duration_behavior_, base::TimeDelta())
+                   .InSecondsF(),
                VelocityBasedDurationBound(old_delta, old_normalized_velocity,
                                           old_duration, new_delta));