Several mojo interface implementation in //ash only support a single
connection. For example, many interfaces exist primarily to provide
a single client/delegate interface back to the browser. Replace
their usage of BindingSet with Binding.

Also clean up some interface overrides where a FooImpl in ash
overrides both mojom::Foo and mojom::FooClient. These impl objects
are not "is-a" client interface impls, so don't derive from that
interface.

Bug: none
Test: ash_unittests, browser_tests
Change-Id: Id65aadda0ad9dd74702fbf21fbb8aa307b39d9fc
Reviewed-on: https://chromium-review.googlesource.com/766649


Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516100}