Remove NullValue() from ComputeTimeInternal

We can get rid of NaN by replacing the return value (double) with an
Optional<double>. This allows us to remove several IsNull() or
ValueOrUnresolved() calls as we can just use the Optional as a boolean.

Last but not least, the NullValue() added in crrev.com/c/1928781 are no longer
needed and thus were removed.

Bug: 737867
Change-Id: I5aa530f58ab648d6dc486122d9a8f8c121f2d057
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929218Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Sergio Villar <svillar@igalia.com>
Cr-Commit-Position: refs/heads/master@{#718562}

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

@@ -257,7 +257,7 @@ class CORE_EXPORT Animation : public EventTargetWithInlineData,
   // is pending is separate from the actual play state.
   AnimationPlayState PlayStateInternal() const;
 
-  double CurrentTimeInternal() const;
+  base::Optional<double> CurrentTimeInternal() const;
   void SetCurrentTimeInternal(double new_current_time);
   void SetCurrentTimeInternal(double new_current_time, TimingUpdateReason);