Use hash set instead of vector for changed RuleSets.

That way, we don't have to consider the same RuleSet multiple times for
invalidation on active stylesheet update. This fixes a regression in
PerformanceTests/CSS/StyleSheetInsert.html which would have been
introduced by

This works because the same style element source text used multiple
times will make us use the same StyleSheetContents from the cache and
hence the same RuleSet for all 50 sheets added in that test. It's a bit
like cheating, but this will also make sure we don't invalidate for the
same RuleSet twice if we re-order stylesheets by removing/inserting a
style element where the CSSStyleSheet pointer will be different, but
the RuleSet stays the same.,

Cr-Commit-Position: refs/heads/master@{#438062}
6 files changed