Bug 1216846 - Don't update hold time when completing a pause if it is already set; r=heycam

The Animation.pause() method operates asynchronously since, if the animation is
currently running on the compositor, we should wait for the animation to stop
on the compositor before establishing the pause time. Otherwise, if the
compositor is ahead of the main thread and we use the main thread's notion of
the current time to establish the pause time, the animation will jump backwards
when we take it off the compositor.

This pause time is represented using the "hold time".

However, when we have a finished animation, its current time is not advancing
but rather its current time is fixed to its end time. This too is represented
using the hold time. As a result, if we pause a finished animation we should
not update its hold time (by calculating the current time from the start time)
but just continue to use the existing hold time. This is true of any other
situation where we might have set the hold time before or during pausing.

dom/animation/Animation.cpp

@@ -933,7 +933,7 @@ Animation::PauseAt(const TimeDuration& aReadyTime)
   MOZ_ASSERT(mPendingState == PendingState::PausePending,
              "Expected to pause a pause-pending animation");
 
-  if (!mStartTime.IsNull()) {
+  if (!mStartTime.IsNull() && mHoldTime.IsNull()) {
     mHoldTime.SetValue((aReadyTime - mStartTime.Value())
                         .MultDouble(mPlaybackRate));
   }

dom/animation/test/css-animations/file_animation-pausing.html

@@ -257,6 +257,19 @@ test(function(t) {
                 'infinite-duration animation');
 }, 'pause() from idle with a negative playbackRate and endless effect');
 
+promise_test(function(t) {
+  var div = addDiv(t, { style: 'animation: anim 1000s' });
+  return div.getAnimations()[0].ready
+    .then(function(animation) {
+      animation.finish();
+      animation.pause();
+      return animation.ready;
+    }).then(function(animation) {
+      assert_equals(animation.currentTime, 1000 * 1000,
+                    'currentTime after pausing finished animation');
+    });
+}, 'pause() on a finished animation');
+
 done();
 </script>
 </body>

dom/animation/test/testcommon.js

@@ -96,7 +96,7 @@ if (opener) {
                         "assert_between_inclusive",
                         "assert_true", "assert_false",
                         "assert_class_string", "assert_throws",
-                        "assert_unreached", "test"]) {
+                        "assert_unreached", "promise_test", "test"]) {
     window[funcName] = opener[funcName].bind(opener);
   }