This change should aid further refactorings. In the future if we need
to support waiting for individual queue idle we can add that back in
a way that's supported by both synchronous and async queue operation.

Bug: b/172704839
Change-Id: I9c9900ac81af60a37719439fe9db6135fcb17d46
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2525146
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>