This will let us squash the attachment types in a follow-up patch.

BUG=angleproject:963

Change-Id: I1efb2e41aa08766189499995b3150aec6fd61c4e
Reviewed-on: https://chromium-review.googlesource.com/263486Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>