This will avoid allocate staging buffer if there isn't one already.

Bug: b/164511310
Change-Id: Ieb5ef12fa58c52c0a62276cab6de135fdd62780c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2377121
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>