This CL also narrows the rw scopes on various call sites.

Bug: chromium:774108,v8:6792
Change-Id: I41a6f5dc4948833baaa441fb998ef40d8a832619
Reviewed-on: https://chromium-review.googlesource.com/758370
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49411}