This reverts commit 25ecc45f.

Reason for revert: Two issues discovered with W^X in V8's 6.5 branch (see v8:7272 and chromium:793428). Still need a way to disable the feature.

Original change's description:
> [heap] Remove --write-protect-code-memory feature flag.
> 
> R=​hpayer@chromium.org
> BUG=v8:6792
> 
> Change-Id: Id3413994de603dac1b7501c6fe376cdac1f9d7ce
> Reviewed-on: https://chromium-review.googlesource.com/866851
> Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50759}

TBR=mstarzinger@chromium.org,hpayer@chromium.org,hablich@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:6792
Change-Id: Ie0d4409b36f22c97a6777e512618beafdef8c2f4
Reviewed-on: https://chromium-review.googlesource.com/883502Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50835}