Only gl::LinkResult remains.

Bug: angleproject:2753
Change-Id: I5e9c68c11453e8ab9db4908451957d7b3db0b110
Reviewed-on: https://chromium-review.googlesource.com/c/1254044Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>