Add GLES 3.1 and 3.2 headers.

Remove GLES 3.0 extension headers and entry point files.

Re-land fixes double define from different style defines in libGLESv2.gypi
and other projects.

Re-land fixes missing defines in angle_util project in gn.

Change-Id: I1952413cd4390e6f5450df809f8f5867bf6f49e2
Reviewed-on: https://chromium-review.googlesource.com/299771Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>