[Code health] Remove blink::NullValue in core/animation--part2

This CL changes a local variable in animation.cc to use
base::Optional<double> so that it removes a usage of
blink::NullValue.

Bug: 791086
Change-Id: Ie82180b853efefb5e0f409c8c17d855c69bd9c59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393072Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805011}

third_party/blink/renderer/core/animation/animation.cc

@@ -1646,9 +1646,9 @@ void Animation::setPlaybackRate(double playback_rate,
   // 4. If previous time is resolved, set the current time of animation to
   //    previous time
   pending_playback_rate_ = base::nullopt;
-  double previous_current_time = currentTime().value_or(Timing::NullValue());
+  base::Optional<double> previous_current_time = currentTime();
   playback_rate_ = playback_rate;
-  if (!Timing::IsNull(previous_current_time)) {
+  if (previous_current_time.has_value()) {
     setCurrentTime(previous_current_time, exception_state);
   }