Store the hash code in 21 bits of the length field.

Change the GetIdentityHash API to be unhandlified, since there's no
property lookup anymore.

Update js/ and test/ to match new API and expections.

Bug: 
Change-Id: I8dc75de4021f59e79b45f3f38ec997c3b3687b24
Reviewed-on: https://chromium-review.googlesource.com/589688
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47259}