While we're at it, we also eliminate the 'unknown' dom depth for the
bloom filter. Computing depth has negligible cost relative to the
amount of work we do setting up the bloom filter at a given depth.
Doing it once per traversal should be totally fine.
I originally separated the elimination of unknown dom depth from the
traversal changes, but I got bloom filter crashes on the intermediate
patch, presumably because I didn't properly fix the sequential traversal
for this case. Given that the final state is green, I just decided to
squash and move on.
We need to do something here to avoid a double-borrow when passing a mutable
StyleContext to match_element. After some discussion on IRC, we decided that
building the bloom filter for this case is probably worthwhile.
This is not a problem for Servo, because we recovered it without much problem a
few lines below, and we never reached the pathological case where the filter has
one element (the root), and we restyled another element with depth 1.
Fixes bug 1321725.
