This patch adds the WebIDL definitions and implementation of
getAnimationPlayers on Element.
It does not include the full definition of AnimationPlayer but only readonly
versions of the currentTime and startTime attributes since these are easy
to implement and enable identifying the different animations that are returned
for the sake of testing.
Web Animations defines getAnimationPlayers as only returning the animations that
are either running or will run in the future (known as "current" animations).
This will likely change since it seems desirable to be able query animations
that have finished but are applying a forwards fill. For now, however, this
patch makes us only return animations that have not finished.
This patch also removes an assertion in ElementAnimation::GetLocalTime that
would fail if called on a finished transition. This assertion is no longer
necessary since an earlier patch in this series removed the overloading of
the animation start time that meant calling this on a finished transition
was unsafe. Furthermore, this assertion, if it were not removed, would fail
if script holds onto a transition and queries its start time after it
completed.
