Bug 1343589 - Add a test for reversing with an infinite target effect end and playback rate of zero r=hiro

We already pass this test but it seemed like a useful test to add since I was
unsure if the early return we have in Animation::Reverse() is valid or not.

MozReview-Commit-ID: J38Euno3VP6

--HG--
extra : rebase_source : ae7f7607189f26385d299003cf48ecd675b615d2

testing/web-platform/meta/MANIFEST.json

@@ -207838,7 +207838,7 @@
    "testharness"
   ],
   "web-animations/timing-model/animations/reversing-an-animation.html": [
-   "9f8e9adb260f4b3cea834bbec6b603c50acbb9fd",
+   "80327fe9a7c68d280b7b4cfba42d081358ee1ea7",
    "testharness"
   ],
   "web-animations/timing-model/animations/set-the-animation-start-time.html": [

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

@@ -131,6 +131,22 @@ test(function(t) {
   assert_equals(animation.playbackRate, 1, 'playbackRate remains unchanged');
 }, 'When reversing throws an exception, the playback rate remains unchanged');
 
+test(function(t) {
+  var div = createDiv(t);
+  var animation = div.animate({}, {duration: 100 * MS_PER_SEC,
+                                   iterations: Infinity});
+  animation.currentTime = -200 * MS_PER_SEC;
+  animation.playbackRate = 0;
+
+  try {
+    animation.reverse();
+  } catch (e) {
+    assert_unreached(`Unexpected exception when calling reverse(): ${e}`);
+  }
+}, 'Reversing animation when playbackRate = 0 and currentTime < 0 ' +
+   'and the target effect end is positive infinity should NOT throw an ' +
+   'exception');
+
 test(function(t) {
   var div = createDiv(t);
   var animation = div.animate({}, {duration: 100 * MS_PER_SEC,