This flag is needed to toggle upcoming variable initialization mode
which uses for loops to optimize the compilation process. Initializing
variables using for loops will be turned on by default, but it needs
to be turned off in Chromium in certain cases. Chromium will use the
flag added in this patch and that code will need to go into Chromium
before finishing the implementation in ANGLE.

BUG=chromium:735497

Change-Id: I3a0e7b7c6cebe60afa72964fbd0caf3b1eafccbc
Reviewed-on: https://chromium-review.googlesource.com/763451
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>