Web-animation: Update CalculateAnimationPlayState to align with the spec.

Previously, the paused_ flag was used to signal that the animation was
in a paused state.  This flag is not part of the spec and is being
phased out.  Instead, we look for pending pause or play tasks and
whether there is a start time to determine if the animation is paused in
alignment with the spec.

Note that fixing play state calculation entailed fixing the handling of
setting a zero playback rate.  Previously, setting the playback rate to
zero would clear the start time; however, this has the unwanted side
effect of forced the animation into the paused state, which is not to
spec.  A running or finished animation should have a start time (if not
a pending change) even if the playback rate is zero.

https://drafts.csswg.org/web-animations/#play-states

Bug: 960944

Change-Id: I1fd43315d3b1c42b22302bf730c450a03836861a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862258
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706628}

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

@@ -344,9 +344,9 @@ void Animation::SetCurrentTimeInternal(double new_current_time,
     if (!hold_time_ || hold_time_ != new_current_time)
       outdated = true;
     hold_time_ = new_current_time;
-    if (paused_ || !playback_rate_) {
+    if (paused_) {
       start_time_ = base::nullopt;
-    } else if (is_limited && !start_time_ &&
+    } else if (is_limited && !start_time_ && playback_rate_ &&
                reason == kTimingUpdateForAnimationFrame) {
       start_time_ = CalculateStartTime(new_current_time);
     }
@@ -762,11 +762,7 @@ Animation::AnimationPlayState Animation::CalculateAnimationPlayState() const {
   //    * both the start time of animation is unresolved and it does not have a
   //      pending play task,
   //    then paused.
-  // TODO(crbug.com/958433): Presently using a paused_ flag that tracks an
-  // animation being in a paused state (including a pending pause). The above
-  // rules do not yet work verbatim due to subtle timing discrepancies on when
-  // start_time gets resolved.
-  if (paused_)
+  if (pending_pause_ || (!start_time_ && !pending_play_))
     return kPaused;
 
   // 3.  For animation, current time is resolved and either of the following

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

@@ -437,7 +437,8 @@ TEST_F(AnimationAnimationTestNoCompositing, StartTimeWithZeroPlaybackRate) {
   animation->setPlaybackRate(0);
   EXPECT_EQ("running", animation->playState());
   SimulateAwaitReady();
-  EXPECT_FALSE(animation->startTime());
+  EXPECT_EQ("running", animation->playState());
+  EXPECT_EQ(0, animation->startTime());
 
   SimulateFrame(10000);
   EXPECT_EQ("running", animation->playState());