This change adds shader type checking before entering
initializeOutputVariables.

BUG: angleproject:2821
Change-Id: Ib931031f2fc187f1f2a1821a09664bbe172a5e90
Reviewed-on: https://chromium-review.googlesource.com/1226229
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>