The EGL extension will allow for querying the cache and populating it
with shaders on startup. The GL extension adds some language to the
spec and allows for querying the program binary cache status of a
Context.

BUG=angleproject:1897

Change-Id: I7233844e41008f097efbb30cf9fe9c89227085fc
Reviewed-on: https://chromium-review.googlesource.com/556383
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>