In some cases we would let the user change the attachments of the
default FBO. This hole in our validation would confound the dEQP
negative API tests (negative_api.buffer.framebuffer_*) and muck
up the remainder of the test run.

BUG=angleproject:901

Change-Id: I2b3c04752d1df3289d41c013cc936eda30e98aad
Reviewed-on: https://chromium-review.googlesource.com/266927Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>