Bug 1360399: Don't deduplicate revalidation selectors. r=bholley

It's unfortunate, but it's a correctness issue. I was looking at the
expectations update here:

 * https://hg.mozilla.org/integration/autoland/rev/659cddddd434

And investigating it I realised that it's wrong to coalesce selectors like that,
because we keep the bloom filter flags.

So in the test cases disabled, we have a selector that looks like this:

msub > :not(:first-child),
msup > :not(:first-child),
msubsup > :not(:first-child),
mmultiscripts > :not(:first-child) {
    -moz-script-level: +1;
    -moz-math-display: inline;
}

And an element that looks like this:

<msubsup><mi></mi><mi></mi></msubsup>

We're only inserting the first selector msub > :not(:first-child) into the set,
so when we're going to match the <mi> elements we fast-reject it in both cases
due to the bloom filter, so they share style.

I can't see an easy way to fix this keeping the deduplication. If we keep it, we
need to remove the bloom filter optimization, which means that we'd trash the
cache for every first-child in the document (the :not(:first-child) effectively
becomes a global rule).

MozReview-Commit-ID: 9VPkmdj9zDg
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/matching.rs

@@ -199,6 +199,8 @@ fn element_matches_candidate<E: TElement>(element: &E,
     debug_assert!(data.has_current_styles());
     let current_styles = data.styles();
 
+    debug!("Sharing style between {:?} and {:?}", element, candidate_element);
+
     Ok(current_styles.primary.clone())
 }