There was a map() without a corresponding unmap().
Add a test which does multiple indexed draws, triggering the problem.

BUG=angleproject:2310

Change-Id: Id33d66f24de2005ec3f9958d33ab4c2630b49dc5
Reviewed-on: https://chromium-review.googlesource.com/875318Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>