Instead of returning a small struct from LinkProgram calls we use
angle::Result. Linking can have 3 cases:

- the link was successful -> angle::Result::Continue
- the link failed -> angle::Result::Incomplete
- there was an internal error -> angle::Result::Stop

Note that any unexpected Incomplete is still an error. Each function
that accepts Incomplete must check explicitly.

This is the last user of ErrorOrResult.

Bug: angleproject:2491
Change-Id: Idba23be27efe4b561720a4bdd8fe486b40779497
Reviewed-on: https://chromium-review.googlesource.com/c/1255645
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@google.com>