[Code health] Remove blink::NullValue in core/animation--part1

This CL changes AnimationEffect::LocalTime to return a
base::Optional<double> instead of double, and so that we
can remove a usage of blink::NullValue.

Bug: 791086
Change-Id: I9908e70b5f5bb329b7fdf15d8d47e2f1973c6c8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393071Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805348}

third_party/blink/renderer/core/animation/animation_effect.h

@@ -99,8 +99,8 @@ class CORE_EXPORT AnimationEffect : public ScriptWrappable {
   AnimationTimeDelta TimeToReverseEffectChange() const {
     return EnsureCalculated().time_to_reverse_effect_change;
   }
-  double LocalTime() const {
-    return EnsureCalculated().local_time.value_or(Timing::NullValue());
+  base::Optional<double> LocalTime() const {
+    return EnsureCalculated().local_time;
   }
 
   const Timing& SpecifiedTiming() const { return timing_; }