We were using a passed in commandBuffer in
ImageHelper::flushStagedUpdates which became invalid during the process
of handling the flush upon which subsequent uses could cause problems
when threading is enabled (and maybe when it's not as well, just harder
to see.)
Have ImageHelper::flushStagedUpdates use the current
OutsideRenderPassCommandBuffer and the code will use the proper one.

Bug: b/168144059
Change-Id: Ib9849efe9366cf61df5e68fd25d17df165dbd3a0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2402354Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>