This allows nsStyleSet::RuleNodeWithReplacement to call it without
constructing an entire (and unnecessary) ElementRuleProcessorData, which
will happen in patch 2.
This patch series (to move SVG Animation rules to their own level of the
cascade) is needed so that they can participate in the mechanism for
updating only animation styles to the present time without touching any
other styles.
In theory, this should have been done as part of 996796 or even as part
of prior work on off main thread animations, except that it doesn't seem
possible to have SVG Animations and CSS Transitions interacting on the
same element and have that element's style out of date, which means it
wasn't necessary for the existing code to update only animation styles.
However, bug 960465 will use this similar mechanism as the new mechanism
for making sure transitions start in a reasonable way (replacing the
mechanism of phases, going from style without animation to style with
animation via nsPresContext::IsProcessingAnimationStyleChange). This
will require that, for SVG Animations to continue interacting correctly
with CSS Transitions, that they have their own cascade level so that
they can participate in RestyleManager::UpdateOnlyAnimationStyles.
Additionally, this is needed even sooner, for bug 977991, which needs
(temporarily, until bug 960465 lands) a mechanism for updating the style
attribute style *and* all animation-phase-dependent styles.
Yes, it's a little bit annoying to have to have another class, and
another object per-document for this, but that's currently what comes
with being a level of the cascade. But it's one class and one object
per document, and I believe there will be substantial benefits to having
all rule processor levels be only-animation-related or
not-animation-related (except for the style attribute level, which is
simple enough that it isn't needed yet).
L. David Baron