Bug 1295842 part 2 - Don't overwrite the hold time when the previous current time is less than zero; r=hiro

This implements the spec change here: ee1577c959

MozReview-Commit-ID: 8XKaocLbrmL

--HG--
extra : rebase_source : 18c402b0af7c21b8721ca796c054a3f58eb21a72

dom/animation/Animation.cpp

@@ -1071,6 +1071,9 @@ Animation::UpdateFinishedState(SeekFlag aSeekFlag,
                currentTime.Value().ToMilliseconds() <= 0.0) {
       if (aSeekFlag == SeekFlag::DidSeek) {
         mHoldTime = currentTime;
+      } else if (!mPreviousCurrentTime.IsNull()) {
+        mHoldTime.SetValue(std::min(mPreviousCurrentTime.Value(),
+                                    TimeDuration(0)));
       } else {
         mHoldTime.SetValue(0);
       }

testing/web-platform/meta/web-animations/timing-model/animations/updating-the-finished-state.html.ini

@@ -1,5 +0,0 @@
-[updating-the-finished-state.html]
-  type: testharness
-  [Updating the finished state when seeking a reversed animation past zero]
-    expected: FAIL
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1295842