For interesting threading behavior need more than two command buffer
helper objects in the pool.

Bug: b/170328907
Change-Id: I394a583f2241ef094abba7a4645a200b9308e466
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2518134
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>