Non-Android platforms are protected in Chrome by site isolation, therefore we can
disable the untrusted code mitigations. We still enable them on Android, and on
simulator builds since that is where we do much of the correctness testing for Android.

Mitigations for ia32 are disabled since they conflict with upcoming work on an ia32
root register.

BUG=chromium:860429, v8:6666

Change-Id: I7831071ec1c4490b7d303314c53c27078fa04979
Reviewed-on: https://chromium-review.googlesource.com/c/1276470Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56753}