This is an intermediate CL as we move to a worker thread.
With the worker thread there will be a pool of more than 2 command
buffers so the current command buffers in use on the main thread
will be pointers that are assigned from that pool.

This CL isolates the command buffers as pointers to make review
simpler.

Bug: b/154030730
Change-Id: I3fc91222b07d5f3bf60f92a4c01b0910daad7df6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2207812
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>