We could handle this case by dropping the animation from the pause-pending table
but that's an extra hashtable lookup that so far seems to be unnecessary.
I have verified that the crashtest added in this patch fails without the code
changes included here.
MozReview-Commit-ID: Ed6u7WRLD2t
The approach here is to lazily check if we have such animations. This allows
animations to be modified after being added to the pending animation tracker
(but not after HasPlayPendingGeometricAnimations is called since we cache the
result at that point) and avoids poor performance when calling
RemovePlayPending.
MozReview-Commit-ID: LRLpCRnzvw
