Convert PushMessaging implementation and clients to new Mojo types

This converts the GetPushMessaging() method from DocumentInterfaceBroker
along with the relevant parts of the implementation of mojom::PushMessaging
at chrome::PushMessagingManager, and the two classes that use it from Blink,
that is, blink::PushProvider and blink::PushMessagingClient.

Note that, for now, this still relies on implicit conversions to get
a PendingReceiver<T> from InterfaceRequest<T> in PushMessagingManager,
which should go away once RenderProcessHostImpl has migrated away
from service_manager::BinderRegistry, to service_manager::BinderMap.

Also, it needs to keep using the InterfaceProvider in PushProvider to bind
the remote interface to avoid thread-related issues that would happen if
we tried to use service_manager::Connector now, since that requires being
in the main thread to run Connect(), and that code usually runs from the
ServiceWorker thread instead. This should stop being a problem once we
have a ServiceWorkers-based InterfaceBroker, at which point the code can
be fully migrated to service_manager::Connector.

Bug: 955171
Change-Id: I62fb6d8b8e1a5872a14740260ea2bca14daf1a15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1655536
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Cr-Commit-Position: refs/heads/master@{#669593}
11 files changed