In this CL, I am simply forwarding the entire API to gr-diff. In
follow-up CLs, I will move the restAPI calls over, but I think it will
be easier to understand the changes when all the forwarding is already
out of the way.
I decided not to touch the styling applied to gr-diff from gr-diff-view
and gr-file-list because that currently does not hurt anyone and seems
less risky.
Bug: Issue 9623
Change-Id: Iacc82198b04b048dd33c380876b1c8b3a80fac95
e5de55e809