Bug 1343589 - Restore the playbackRate when reverse() throws. r=hiro

MozReview-Commit-ID: FNIZK9yQWyV

--HG--
extra : rebase_source : 32cdcc52ccd8192addc0ed2239f74eb109970597

dom/animation/Animation.cpp

@@ -536,6 +536,13 @@ Animation::Reverse(ErrorResult& aRv)
   SilentlySetPlaybackRate(-mPlaybackRate);
   Play(aRv, LimitBehavior::AutoRewind);
 
+  // If Play() threw, restore state and don't report anything to mutation
+  // observers.
+  if (aRv.Failed()) {
+    SilentlySetPlaybackRate(-mPlaybackRate);
+    return;
+  }
+
   if (IsRelevant()) {
     nsNodeUtils::AnimationChanged(this);
   }

testing/web-platform/meta/web-animations/timing-model/animations/reversing-an-animation.html.ini

@@ -1,5 +0,0 @@
-[reversing-an-animation.html]
-  type: testharness
-  [When reversing throws an exception, the playback rate remains unchanged]
-    expected: FAIL
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1343589