Emulate gl_FragColor broadcasting behavior when GL_EXT_draw_buffers is enabled. BUG=angleproject:1467,635433 TEST=WebGL conformance, angle_unittests Change-Id: I9eb4ce715732087a3786da886f42243716f2b9b2 Reviewed-on: https://chromium-review.googlesource.com/367532 Commit-Queue: Zhenyao Mo <zmo@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>