Fixes:
dEQP-GLES2.functional.shaders.preprocessor.invalid_function_definitions.unique_param_name_vertex
dEQP-GLES2.functional.shaders.preprocessor.invalid_function_definitions.unique_param_name_fragment

BUG=angleproject:989

Change-Id: I32198f1c9036c371b46e7ad2986a44e42fd38e20
Reviewed-on: https://chromium-review.googlesource.com/267396Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>