update now-incorrect license details for Mesa headers

We no longer use any LGPL code from Mesa, so let's remove that license
from the corresponsing README.chromium file.

Also, GLES/glext.h, GLES/gl.h and GLES/glplatform.h are made available
under another license that is not already mentioned- "SGI Free Software
B License Version 2.0". Let's add this license text.

Note that the reference URL for this license is now a broken link, and
these header files do not include the actual license themselves. The
license text added by this patch was taken from this (reputable) source:
https://spdx.org/licenses/SGI-B-2.0.html

BUG=873321

Change-Id: I324fe79c070a996bcab9ff78021f45572d6bbf39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1813353
Commit-Queue: Mostyn Bramley-Moore <mostynb@vewd.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709267}

third_party/mesa_headers/README.chromium

 Name: mesa_headers
 Version: 9.0.3
 URL: http://www.mesa3d.org/
-License: MIT and LGPL v2
+License: MIT and SGI Free Software B License Version 2.0
 Security Critical: Yes
 
 Description:
 This directory contains a copy of the Mesa headers.
 
-The license file in this directory is derived from src/docs/license.html
-and src/docs/COPYING.
+The LICENSE file in this directory is derived from those header files.
 
 Modifications made:
 - Added the file README.chromium (this file)