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: I3763b8ca38b1e14ee8456a54592c01e0fd89692c
Reviewed-on: https://chromium-review.googlesource.com/286101Tested-by: Kimmo Kinnunen <kkinnunen@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>