Add flag for turning off initializing variables with loops 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>