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Previously we made CastMessage & CastSessionImpl (now
CastSessionController) lifecycle bound to per connection, which
makes the session management over-complicated. This CL makes
those classes having the same lifecycle as CafBaseMRP/CafMRP, so
there's no need to create/set/unset instances any more. These
instances will only be active when there's currently an active
session.

Bug: 711860
Change-Id: I1736bb7085cd1ffbffed8ec40f3410ebe997e64e
Reviewed-on: https://chromium-review.googlesource.com/1171582Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Zhiqiang Zhang <zqzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584309}