Turns on --harmony-hashbang when experimental JS features are enabled.

BUG=v8:8523
R=gsathya@chromium.org, adamk@chromium.org

Change-Id: I2b1e6c146d900cb8a998060b4b93127f907b6728
Reviewed-on: https://chromium-review.googlesource.com/c/1417382Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#58933}