In WebGL one can Clear, DrawArrays, and DrawElements to a framebuffer
with a missing attachment with no error.

BUG=angleproject:1822

Change-Id: I4dece2fa8fad31c812e24ae18bdc380c2857a1f8
Reviewed-on: https://chromium-review.googlesource.com/502967Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>