Fixes angle_end2end_test BlendMinMaxTest with the blend_minmax extension
enabled.

BUG=angleproject:883

Change-Id: Iae2441d11a3c4c497a1ddc4d250120fd85175d52
Reviewed-on: https://chromium-review.googlesource.com/274802Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>