style: Set the important rule change flag when a newly important rule is inserted.

Without this change an assertion checking IsInStyleRefresh() in
EffectCompositor::PostRestyleForAnimation will be hit when we call
FindAnimationsForCompositor from RestyleManager::DoProcessPendingRestyles
that will be introduced in a subsequent commit in this series.

I wrote a crash test which causes an assertion in KeyframeEffect::CanThrottle()
without the subsequent commit, but we need more work in display item stuff to
make the assertion won't happen (bug 1508466).

Differential Revision: https://phabricator.services.mozilla.com/D12368

components/style/rule_tree/mod.rs

@@ -453,6 +453,7 @@ impl RuleTree {
                         StyleSource::from_declarations(pdb.clone_arc()),
                         level,
                     );
+                    *important_rules_changed = true;
                 }
             } else {
                 if pdb.read_with(level.guard(guards)).any_normal() {