Rewrite the style sharing candidate cache.

The style candidate cache had regressed a few times (see #12534), and my
intuition is that being able to disable all style sharing with a single rule in
the page is really unfortunate.

This commit redesigns the style sharing cache in order to be a optimistic cache,
but then reject candidates if they match different sibling-affecting selectors
in the page, for example.

So far the numbers have improved, but not so much as I'd wanted (~10%/20% of
non-incremental restyling time in general). The current implementation is really
dumb though (we recompute and re-match a lot of stuff), so we should be able to
optimise it quite a bit.

I have different ideas for improving it (that may or may not work), apart of the
low-hanging fruit like don't re-matching candidates all the time but I have to
measure the real impact.

Also, I need to verify it against try.

components/style/sequential.rs

@@ -35,5 +35,6 @@ pub fn traverse_dom<N, C>(root: N,
     if context.should_process(root) {
         doit::<N, C>(&context, root);
     }
+    // Clear the local LRU cache since we store stateful elements inside.
+    context.local_context().style_sharing_candidate_cache.borrow_mut().clear();
 }
-