Create context and validation stubs for unimplemented extensions. This
will allow us to use an autogeneration script in the future to generate
entry_points_gles_2_0_ext.cpp.

Bug:angleproject:2263
Change-Id: If3ad3ebd823d65085c7f143cce9e09187e65d4da
Reviewed-on: https://chromium-review.googlesource.com/998440Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>