Remove the --extra-masking and --mask-array-index flags. Instead, use
--untrusted-code-mitigations directly.
This also changes the default for these flags: There were off by
default so far, but --untrusted_code_mitigations is on by default.

Store the value of the untrusted_code_mitigations flag in the
CompilationInfo in order to ensure that it stays consistent during the
compilation of one function.

R=jarin@chromium.org, bmeurer@chromium.org, hablich@chromium.org
CC=rmcilroy@chromium.org

Bug: chromium:798964
Change-Id: I15a919e741f0628afa6a6ea1e8274ad0c4399929
Reviewed-on: https://chromium-review.googlesource.com/850412
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Hablich <hablich@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50373}