A common place to define required GL includes gives us a nice
point to centralize GL customizations. In the header currently
are the basic GLES headers with extensions, and a define
carried over from desktop GL.

BUG=angle:466

Change-Id: I6fc61947b4514654ec21355a786904eac04656c0
Reviewed-on: https://chromium-review.googlesource.com/204936Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>