KHR_robustness says that certain queries that poll such as GetSynciv with
SYNC_STATUS should still generate errors after context loss but also return
that the result is available.

BUG=angleproject:3379

Change-Id: Ibf61f8481bc7c3d1b4fa8979dc404d0ca3952fcd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1566142
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>