This speeds up insertion and removal by reducing the time taken to hash.

Bug: v8:6992
Change-Id: I4e9e1c97a5503bf5a09c2428980a3c2d6bf4051e
Reviewed-on: https://chromium-review.googlesource.com/735419Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48867}