This introduces {Heap::write_protect_code_memory} as a race-free copy of
the underlying {FLAG_write_protect_code_memory} flag. Since this flag is
checked from the parallel sweeper, subsequent flag implications might be
racing against the read. This ensures race-free reads.

R=hpayer@chromium.org
BUG=v8:6792,chromium:774108,v8:7106

Change-Id: I1a1073f11e91bebd60f8d5da440845452ec67c50
Reviewed-on: https://chromium-review.googlesource.com/781662Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49535}