Vulkan: Increase CommandBufferHelper pool size 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>