In some cases, e.g. mouse event, we need to request an animation-only restyle
to force flush all throttled animations when we handle an event with
coordinates. This restyle shouldn't trigger a normal restyle, and add a
different traversal flag, TraversalRestyleBehavior::ForAnimationOnly, to check.
Allow -moz-stack-sizing to work in a single direction only.
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https://bugzilla.mozilla.org/show_bug.cgi?id=1364115https://reviewboard.mozilla.org/r/138434/
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We need another flag that represents allow-negative-number for SMIL, so
this enum will also comprise the another parsing mode that allows negative number.
