Add #!/usr/bin/python2 and the executable permission bit to all scripts
where missing.

BUG=angleproject:2209

Change-Id: Ib33017c17e579c371b89bbfbdb7136b870027dc5
Reviewed-on: https://chromium-review.googlesource.com/862987Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>