Fixes conformance/renderbuffers/framebuffer-object-attachment.html

BUG=angleproject:884

Change-Id: I421b563fa8549eb2edc42a9ac1c798fc457256ee
Reviewed-on: https://chromium-review.googlesource.com/275690Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>