The behavior is undefined in GL ES when multiple attachments are used
with gl_FragColor but WebGL clarifies that:
 - If the extension is enabled, broadcast happens
 - If the extension isn't enabled, only the first attachment is written

BUG=angleproject:2048

Change-Id: I6d85ba91df77d42fb8863a03f1faa006cd7817bf
Reviewed-on: https://chromium-review.googlesource.com/523809Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>