From the 3.0 spec (Section 3.1):

"When enabled, RASTERIZER_DISCARD also causes the Clear and
ClearBuffer* commands to be ignored."

BUG=angleproject:1227

Change-Id: I0316103827927e8af9732d7b4565d787b881ccb8
Reviewed-on: https://chromium-review.googlesource.com/313993
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>