fix(script): update animation timeline before processing pending events

This commit reverses the order of the `send_pending_events` and
`update_animation_timeline` calls in `ScriptThread::update_animations_
send_events` so that animation-related events pended by the latter are
processed by the former.

The new calling order is more compliant with the "update animations and
send events" algorithm steps from [the Web Animations specification][1].

The old implementation was prone to blocking for an indefinite period
while holding pending events. Due to complex interaction with other
events and timing behavior, I was only able to reproduce the problem
under the following conditions:

- *The `maybe_observe_paint_time` call in `LayoutThread::compute_abs_
  pos_and_build_display_list` is removed from the code*. While
  performance events may seem irrelevant to the issue, they would
  bombard the script thread with events. *Any* extra messages received
  would unblock the event loop and prevent the manifestation of the
  issue. (But, of course, we aren't supposed to count on that to avoid
  the issue.)

- Servo is running in a headless mode, which somehow makes it less
  likely for the script thread to receive a `TickAllAnimations` event
  after sending `AnimationState::NoAnimationsPresent`.

With the above conditions met and the stars aligned, you can reproduce
the problem by running the WPT test `/css/css-transitions/events-001.
html`.

[1]: https://drafts.csswg.org/web-animations-1/#update-animations-and-send-events

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