We now create empty primary command buffers when there are
no existing commands to submit. The main reason to remove this
is so that we don't have to make a synchronous call to
CommandQueue or the async command processor to determine if there
are queued primary commands.

Bug: b/172704839
Change-Id: I53f65eaa187b618e4ed82a5d2805a76e6a9e036c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2524547
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>