Reuse flushing logic in finish() as there's an increasing amount of logic
needed to submit work in order that would otherwise be duplicated.

Bug: angleproject:3289

Change-Id: Ia489e285615ebaa48b01d8a202871bebf7de0ac2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1623813Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>