This extension is mandatory for EXT_map_buffer_range support. We can
emulate it using GLES 3.0 core map functionality.

BUG=angleproject:1751

Change-Id: Idba09ce7276603d5556039f4a49aa0b87cae22aa
Reviewed-on: https://chromium-review.googlesource.com/431826Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>