Reland #1: Clean up interface of attachment broker.
I forgot to initialize a member variable, which is why the original CL flakily
succeeded (and managed to pass the CQ).
Original issue's description:
> ipc: Clean up interface of attachment broker.
>
> AttachmentBrokerUnprivileged now has a method
> DesignateBrokerCommunicationChannel which is used by non-broker processes to
> designate an IPC::Channel as the communication medium for brokerable attachment
> messages. IPC::Channel has a new member attachment_broker_endpoint_ which causes
> it to pass all messages through the attachment broker before forwarding them to
> the listener.
>
> BUG=493414
>
> Committed: https://crrev.com/9a06836982214f2edced21bbd1615b49e7f231bf
> Cr-Commit-Position: refs/heads/master@{#341143}
BUG=493414
TBR=tsepez@chromium.org
Review URL: https://codereview.chromium.org/1258323004
Cr-Commit-Position: refs/heads/master@{#341207}
14 files changed
