Otherwise we could potentially be destroying resources that are
currently in use.

Was caught by changing some of the test tear down ordering. Also might
affect some of the dEQP EGL multithread tests.

Requires fixing the serial tracking of the "null" buffer. Otherwise we
could be destroying the null buffer in the incorrect order.

Bug: angleproject:3261
Bug: angleproject:3059
Change-Id: I57d1059ce9b1da19ced8ac753fab80c625901066
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1531291Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>