Turns --harmony-hashbang on by default.

Intent to ship: https://groups.google.com/d/msg/v8-dev/hlCVa_XZ3TM/UWjjyOq3FwAJ
ChromeStatus page: https://www.chromestatus.com/feature/5134505706782720

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

Change-Id: I821f69e45eb0a63a3f49181e2b88b0bcd091af2c
Reviewed-on: https://chromium-review.googlesource.com/c/1486113Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#59847}