Import eglextchromium.h from Chromium.

Import eglextchromium.h from Chromium.

In order to support the (currently unregistered) Chromium-specific EGL
extension eglGetSyncValuesCHROMIUM on Intel systems, we need to import
the Chromium header that defines it.  The file was downloaded from

https://chromium.googlesource.com/chromium/chromium/+/trunk/ui/gl/EGL/eglextchromium.h

It is subject to the license found at

https://chromium.googlesource.com/chromium/chromium/+/trunk/LICENSE

I have imported the header file and added the license text to the top.
The only change was to fix the include guard on the Chromium header to
change the last line from a #define to a #endif, which makes the header
actually compile.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Cc: Jamey Sharp <jamey@minilop.net>
Cc: Ian Romanick <idr@freedesktop.org>
Cc: Stéphane Marchesin <stephane.marchesin@gmail.com>