This allows us to enable these tests:
dEQP-GLES2.functional.shaders.fragdata.valid_static_index

Bug: angleproject:2594
Change-Id: I2c9a10063500d96d1c4bfb761f4b45590114b7ef
Reviewed-on: https://chromium-review.googlesource.com/1100957Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>