Add GLES 3.1 and 3.2 headers.

Remove GLES 3.0 extension headers and entry point files.

Change-Id: I8e9df6262dc1b2536a409c5791734e4a4d63b115
Reviewed-on: https://chromium-review.googlesource.com/299341
Tryjob-Request: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>