Add support for animationend event

This is triggered when an animation finishes. This is a high priority
because it allows us to start rooting nodes with animations in the
script thread.

This doesn't yet cause a lot of tests to pass because they rely on the
existence of `Document.getAnimations()` and the presence of
`animationstart` and animationiteration` events.

tests/wpt/mozilla/meta/MANIFEST.json

@@ -13870,7 +13870,7 @@
      ]
     ],
     "interfaces.html": [
-     "1a579837cc22d31a7792566615d9e321b3d7fe39",
+     "b6034be26af3c2edd1ef41703857fa99bd2cd639",
      [
       null,
       {}