This reverts commit bf19e60c.

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:
> [platform] Remove {PageAllocator::kReadWriteExecute}.
> 
> Now that write-protection of code memory is enabled everywhere and V8 is
> fully W^X compliant, we can remove the permission mode in question.
> 
> R=​hpayer@chromium.org
> BUG=v8:6792
> 
> Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
> Change-Id: I80fe95ac6bb0e2d1ad6d993154ce45d492d941be
> Reviewed-on: https://chromium-review.googlesource.com/866855
> Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Reviewed-by: Bill Budge <bbudge@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50770}

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

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

Bug: v8:6792
Change-Id: If4a205497ac83084a4092560363affb13b391462
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/883461Reviewed-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@{#50834}