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.
