Intent to implement and ship: animationcancel event

[Mantaroh Yoshinaga]

Summary:
Adds a new event that fires when an animation is terminated without
finishing normally (e.g. element is made display:none, animation-name
property is updated etc.). This parallels the recently added
transitioncancel event and allows authors to know that they should not
keep waiting for an animationend event.

Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1302648

Link to standard:
https://drafts.csswg.org/css-animations/#eventdef-
animationevent-animationcancel

Platform coverage:
All platforms

Estimated or target release:
Firefox 54

Preference behind which this will be implemented:
No pref

DevTools bug:
No special Devtools handling required.

Do other browser engines implement this?
No.
Blink has a bug on file for implementing transitioncancel[1] which
is marked "Hotlist-Interop" and we expect they will implement
animationcancel at roughly same time.
[1] https://bugs.chromium.org/p/chromium/issues/detail?id=642487

Tests:
The working-in-progress patch include tests in web-platform-tests
(testharness.js) format. However, we are still waiting for the CSSWG to
merge their test repository into web-platform-tests before submitting
them there.