Exposing the existing Context::AllowCodeGenerationFromStrings(false) API
to the command line.

Bug: v8:7134
Change-Id: I062ccff0b03c5bcf6878c41c455c0ded37a1d743
Reviewed-on: https://chromium-review.googlesource.com/809631Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49911}