Default uniforms are gathered in a uniform block, so treat them like
other nameless uniform block fields.

Bug: angleproject:4889
Change-Id: If340040ab4c59f3243c40de7daba530b2289e147
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2939333Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>