Refactors the gl::FenceNV and gl::Sync classes.

Bug: angleproject:2491
Change-Id: I0fe73d1ccf5407f460e173a3061735b330a88511
Reviewed-on: https://chromium-review.googlesource.com/c/1289712Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>