[animation-trigger] Repeat triggers should not cancel the animation

Currently, repeat triggers cancel their animations. This isn't the
right behavior. Repeat triggers ought to reset their animations to the
effect's before phase.

This patch is a step in that right direction which, instead of
cancelling the animation, sets the current time of the animation and
pauses it. This patch partially introduces the notion of
`pause_for_trigger_` which is a necessary bit to signify that the
animation is paused in anticipation of a trigger event. A future patch
will implement the pause_for_trigger_ semantics more fully by factoring
it into decisions about the effect's timing & phase.

animation-trigger-repeat-tentative.html now reflects that the animation
is not canceled but simply reset to its initial state.

Bug: 410602051, 390314945
Change-Id: I679f41b070140a2ae6aa44f4dce51c259b397c9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6579523
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: David Awogbemila <awogbemila@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466090}