Clear mAutoAlignStartTime when transitioning away from a finite timeline.

mAutoAlignStartTime is only meaningful for finite timelines: it is set in
the toFiniteTimeline branch of Animation::SetTimelineNoUpdate, and the
AutoAlignStartTime procedure asserts that the current timeline is not
monotonically increasing.

The other branch in SetTimelineNoUpdate only runs SetCurrentTimeNoUpdate
(which clears mAutoAlignStartTime as a side effect via SilentlySetCurrentTime)
when previous progress is resolved. For a pause-pending scroll-driven
animation whose start time was never auto-aligned -- mStartTime null,
mHoldTime null, so previousCurrentTime and previousProgress are both null --
that branch is skipped and the animation lands on the new (potentially
monotonic) timeline still carrying mAutoAlignStartTime = true.

Per spec [1], the fromFiniteTimeline branch only acts when previous
progress is resolved and says nothing about the unresolved case. We
deviate by unconditionally clearing mAutoAlignStartTime in that branch,
because otherwise the flag's invariant (only true while the timeline is
finite) is violated and AutoAlignStartTime would later fire on a
monotonic timeline.

Clear the flag unconditionally in the fromFiniteTimeline branch so the
invariant holds regardless of previousProgress, and keep the existing
SetCurrentTimeNoUpdate call nested inside the previous-progress check.

[1] https://drafts.csswg.org/web-animations-2/#setting-the-timeline

Differential Revision: https://phabricator.services.mozilla.com/D300738

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2031862
gecko-commit: cf7a1644bfc0a64533b3fc427431a01d55fcc630
gecko-commit-git: a8588405288dd8fa3b173faf25cde9e1c32cfde3
gecko-reviewers: layout-scroll-driven-animation-reviewers, boris

Author: hiikezoe

scroll-animations/crashtests/change-timeline-from-finite-to-monotonic-crash.html

@@ -0,0 +1,16 @@
+<style>
+@keyframes anim {}
+</style>
+<script>
+document.addEventListener("DOMContentLoaded", () => {
+  const a = document.createElement("style")
+  const b = document.createTextNode("* { animation-play-state: paused; animation-timeline: scroll(), none, --")
+  a.appendChild(b)
+  document.documentElement.appendChild(a)
+  const c = document.createElementNS("http://www.w3.org/2000/svg", "font-face")
+  document.documentElement.appendChild(c)
+  c.style.animationName = '"anim", none, none'
+  self.scroll({"left": 4096})
+  b.data = ""
+})
+</script>