getOutsideRenderPassCommandBuffer returns the command buffer directly
since it's now stateless and cannot throw an error. All the RenderPass
begin/end and flushing are done by the dependency functions (buffer/
image read/write).

Bug: angleproject:4911
Change-Id: I5e7806be9d0e1b5e358524bd485298d660fac942
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2339544
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>