Rewrite test in terms of promise rejection

This patch rewrites animation-finish-event-cancelled.html to listen for
rejection of the animation's finished promise instead of using a combination
of onfinish and assertions about code being unreached. This makes the test
easier to read and brings it more in line with the style of tests in the
web-platform-tests suite.

BUG=623437

Review-Url: https://codereview.chromium.org/2145263002
Cr-Commit-Position: refs/heads/master@{#405384}
1 file changed