Web Animations: use WTF::Optional for StartAnimationOnCompositor's start_time

Previously nullable values in the animations code were tracked by
storing nulls as quiet_NaN() and using std::isnan as a null-detector.
It is much more explicit to store such values using WTF::Optional, which
forces code to consider whether or not the offset exists.

Bug: 791086
Change-Id: Ice80a1d3d686b0ff1f0bc2a1f3eac2eff51a8e38
Reviewed-on: https://chromium-review.googlesource.com/981076Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546145}

third_party/WebKit/Source/core/animation/Animation.cpp

@@ -906,19 +906,19 @@ void Animation::StartAnimationOnCompositor(
 
   bool reversed = playback_rate_ < 0;
 
-  // TODO(crbug.com/791086): Make StartAnimationOnCompositor use WTF::Optional.
-  double start_time = NullValue();
+  WTF::Optional<double> start_time = WTF::nullopt;
   double time_offset = 0;
   if (start_time_) {
     start_time = TimelineInternal()->ZeroTime() + start_time_.value();
     if (reversed)
-      start_time -= EffectEnd() / fabs(playback_rate_);
+      start_time = start_time.value() - (EffectEnd() / fabs(playback_rate_));
   } else {
     time_offset =
         reversed ? EffectEnd() - CurrentTimeInternal() : CurrentTimeInternal();
     time_offset = time_offset / fabs(playback_rate_);
   }
 
+  DCHECK(!start_time || !IsNull(start_time.value()));
   DCHECK_NE(compositor_group_, 0);
   ToKeyframeEffect(content_.Get())
       ->StartAnimationOnCompositor(compositor_group_, start_time, time_offset,

third_party/WebKit/Source/core/animation/CompositorAnimations.cpp

@@ -379,7 +379,7 @@ void CompositorAnimations::CancelIncompatibleAnimationsOnCompositor(
 void CompositorAnimations::StartAnimationOnCompositor(
     const Element& element,
     int group,
-    double start_time,
+    WTF::Optional<double> start_time,
     double time_offset,
     const Timing& timing,
     const Animation* animation,
@@ -573,7 +573,7 @@ void AddKeyframesToCurve(PlatformAnimationCurveType& curve,
 void CompositorAnimations::GetAnimationOnCompositor(
     const Timing& timing,
     int group,
-    double start_time,
+    WTF::Optional<double> start_time,
     double time_offset,
     const KeyframeEffectModelBase& effect,
     Vector<std::unique_ptr<CompositorKeyframeModel>>& keyframe_models,
@@ -644,8 +644,8 @@ void CompositorAnimations::GetAnimationOnCompositor(
     std::unique_ptr<CompositorKeyframeModel> keyframe_model =
         CompositorKeyframeModel::Create(*curve, target_property, group, 0);
 
-    if (!std::isnan(start_time))
-      keyframe_model->SetStartTime(start_time);
+    if (start_time)
+      keyframe_model->SetStartTime(start_time.value());
 
     keyframe_model->SetIterations(compositor_timing.adjusted_iteration_count);
     keyframe_model->SetIterationStart(compositor_timing.iteration_start);

third_party/WebKit/Source/core/animation/CompositorAnimations.h

@@ -101,7 +101,7 @@ class CORE_EXPORT CompositorAnimations {
                                                        const EffectModel&);
   static void StartAnimationOnCompositor(const Element&,
                                          int group,
-                                         double start_time,
+                                         WTF::Optional<double> start_time,
                                          double time_offset,
                                          const Timing&,
                                          const Animation*,
@@ -137,7 +137,7 @@ class CORE_EXPORT CompositorAnimations {
   static void GetAnimationOnCompositor(
       const Timing&,
       int group,
-      double start_time,
+      WTF::Optional<double> start_time,
       double time_offset,
       const KeyframeEffectModelBase&,
       Vector<std::unique_ptr<CompositorKeyframeModel>>& animations,

third_party/WebKit/Source/core/animation/CompositorAnimationsTest.cpp

@@ -153,9 +153,9 @@ class AnimationCompositorAnimationsTest : public RenderingTest {
       StringKeyframeEffectModel& effect,
       Vector<std::unique_ptr<CompositorKeyframeModel>>& keyframe_models,
       double animation_playback_rate) {
-    CompositorAnimations::GetAnimationOnCompositor(
-        timing, 0, std::numeric_limits<double>::quiet_NaN(), 0, effect,
-        keyframe_models, animation_playback_rate);
+    CompositorAnimations::GetAnimationOnCompositor(timing, 0, WTF::nullopt, 0,
+                                                   effect, keyframe_models,
+                                                   animation_playback_rate);
   }
 
   bool DuplicateSingleKeyframeAndTestIsCandidateOnResult(

third_party/WebKit/Source/core/animation/KeyframeEffect.cpp

@@ -232,7 +232,7 @@ KeyframeEffect::CheckCanStartAnimationOnCompositor(
 
 void KeyframeEffect::StartAnimationOnCompositor(
     int group,
-    double start_time,
+    WTF::Optional<double> start_time,
     double current_time,
     double animation_playback_rate,
     CompositorAnimation* compositor_animation) {

third_party/WebKit/Source/core/animation/KeyframeEffect.h

@@ -101,7 +101,7 @@ class CORE_EXPORT KeyframeEffect final : public AnimationEffect {
       double animation_playback_rate) const;
   // Must only be called once.
   void StartAnimationOnCompositor(int group,
-                                  double start_time,
+                                  WTF::Optional<double> start_time,
                                   double time_offset,
                                   double animation_playback_rate,
                                   CompositorAnimation* = nullptr);

third_party/WebKit/Source/modules/animationworklet/WorkletAnimation.cpp

@@ -289,7 +289,7 @@ bool WorkletAnimation::StartOnCompositor(String* failure_message) {
   CompositorAnimations::AttachCompositedLayers(target,
                                                compositor_animation_.get());
 
-  double start_time = std::numeric_limits<double>::quiet_NaN();
+  WTF::Optional<double> start_time = WTF::nullopt;
   double time_offset = 0;
   int group = 0;