GL_ARB_draw_buffers adds special semantics when writing to gl_FragData even
if multiple renderbuffers is part of the core spec so it needs to be declared
in the translated shader.

BUG=617410

Change-Id: Id5524634c278ba303413b11251d560744f2517ca
Reviewed-on: https://chromium-review.googlesource.com/350121Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>