Fixes:
dEQP-GLES2.functional.shaders.preprocessor.definitions.undefine_object_invalid_syntax_vertex
dEQP-GLES2.functional.shaders.preprocessor.definitions.undefine_object_invalid_syntax_fragment
dEQP-GLES2.functional.shaders.preprocessor.invalid_definitions.undef_non_identifier_2_vertex
dEQP-GLES2.functional.shaders.preprocessor.invalid_definitions.undef_non_identifier_2_fragment

BUG=angleproject:989

Change-Id: I279a38aaae8010017ef6e3f1aa139ae03f374680
Reviewed-on: https://chromium-review.googlesource.com/267397Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>