glGetQueryObject* requires forward progress in the queue regardless of
whether we are waiting on the result or busy-looping over whether the
results are available.  This commit calls flush() if the query has
pending work.

Additionally, this fixes a race condition where glGetQueryObject* may be
accessing a query whose corresponding batch has been submitted but not
yet executed.  In such a case, the GPU may not have already reset the
query, so we have to wait on the fence of that batch to make sure the
query results are reliably available.

Bug: angleproject:2855
Change-Id: I977909c6526c0778a13722a8b8b73e54ad0202f6
Reviewed-on: https://chromium-review.googlesource.com/c/1279125
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>