Add EGL_CHROMIUM_get_sync_values extension text

In order to have something for implementors to look at, let's add
this extension here. We can always replace it with a newer version
once we have everything specced.

BUG=chromium:366935
TEST=no test

Review URL: https://codereview.chromium.org/478453003

git-svn-id: http://src.chromium.org/svn/trunk/src/gpu@289893 4ff67af0-8c30-449e-8e8b-ad334ec8d88c