Fixes:
 * conformance/glsl/misc/empty_main.vert.html
 * conformance/glsl/misc/gl_position_unset.vert.html

BUG=angleproject:882

Change-Id: I92a465b58f319b14e4f697d9bb5b45427a6247c0
Reviewed-on: https://chromium-review.googlesource.com/273548Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>