Handle null effect when interrupting a transition

If the effect of a CSS Transition is removed (via the getAnimations API)
and then the transition is interrupted by removing the 'to' style, we
previously assumed that the effect would always exist. This CL adds a
null-check for that case.

Bug: 992668
Change-Id: I399be7fd859bac0ca6e7ee1e29dbcbbae2e89403
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1753671
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686932}
2 files changed