This will need to be matched with a corresponding browser-side CL.
It will enable writing out binary shaders to disk.

BUG=angleproject:1897

Change-Id: I443281086050b9711b92a034cf37f808dd919007
Reviewed-on: https://chromium-review.googlesource.com/542963Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>