In many cases, this is performance neutral because we can fall off 
the JavaScript monomorphic fast path for the hash lookup.
Once we store the hash code in known fixed position, this should
get much faster.

Change-Id: I3bb52ee6482fe2c35b7abe70c1d0c21935c6cc1d
Reviewed-on: https://chromium-review.googlesource.com/566679
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46692}