Will need access to CommandBatch class in threading worker.

Bug: b/154030730
Change-Id: Ia79eab77a81b135c22bdeecbaf65bf3c301dc987
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2447442Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>