Binding to GL_FRAMEBUFFER is supposed to set the read and draw framebuffer
bindings instead of being a separate binding point.

BUG=angleproject:885

Change-Id: Ic6d9056a2f8bfa472587c5ed030c15fcc93574c7
Reviewed-on: https://chromium-review.googlesource.com/267460Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>