Passes all tests in:
 * dEQP-GLES3.functional.occlusion_query
 * angle_end2end_tests

BUG=angleproject:887

Change-Id: I643ab4c28cb545de9e7b0e1740e3fd8e2aa9d3d9
Reviewed-on: https://chromium-review.googlesource.com/302338Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>