Don't use ErrorOrResult. Also pass gl::Context around to more places.

Bug: angleproject:2378
Change-Id: Id498471fc9ab4f4d6b2285e110381b10a1247689
Reviewed-on: https://chromium-review.googlesource.com/1148465Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>