Preserve running CSS Animations across changes to animation styles according to same name index

We currently only keep track of the last animation that used
a given @keyframes name. If multiple animations use the same
@keyframes rule we fail to update their timing input correctly
as animation styles are updated.
This change makes us track animation-name references to running
animations based on name and name index (nth occurrence of name)
rather than just name.

This behaviour does not conform to any spec text as the
specification does not dictate what to do in this scenario:
"Need to specify better handling of dynamic changes of animation-name"
https://drafts.csswg.org/css-animations-1/#other-open-issues

The behaviour introduced in this patch is arbitrary though
better than the existing behaviour of ignoring running
animations and crashing debug on timing updates.

BUG=487092

Review URL: https://codereview.chromium.org/1402143004

Cr-Commit-Position: refs/heads/master@{#354457}

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