Make all fragment shader out variables require location layout
qualifier. Previously, the last variable did not need a location layout
qualifier if the previous variables had those.

TEST=angle_unittests
BUG=angleproject:1070

Change-Id: Ifb66ee52b811409f5278eaad330d6cd9b8ea059f
Reviewed-on: https://chromium-review.googlesource.com/287100Tested-by: Kimmo Kinnunen <kkinnunen@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>