[Cast MRP] Implement leave_session.

leave_session will cause the originating client as well as other clients
with matching origin and/or tab ID (depending on autojoin policy) to
leave the session.

Bug: 809249
Change-Id: Ia0a85c09b691cb4ed41bef65d35fe511ac6cacf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1432573
Commit-Queue: John Williams <jrw@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641230}
8 files changed