Intel GPUs enter an infinite loop unless told to flush.

BUG=angleproject:1038

Change-Id: I7c3d9dd08bf1ee09cb163dd859f817379a72cd65
Reviewed-on: https://chromium-review.googlesource.com/274942Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>