This works around a bug in the libc++ implementation of bitset
(https://bugs.llvm.org/show_bug.cgi?id=35438) resulting in high
bits outside the bitset leaking through, breaking the ordering
invariant of PersistentMap::iterator. This did not surface so far
because the hash values used in escape analysis so far all only used
32 bits.

Bug: 
Change-Id: I18ce703020bf1fb3e1b412edaa899fa1afe0bba0
Reviewed-on: https://chromium-review.googlesource.com/793613
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50684}