Change ChannelManager to use unique_ptr

Also clarify the ownership of created channels.

Bug: None
Change-Id: I9cbaec177069d27da2b3b48b93af48f705243b4b
Reviewed-on: https://chromium-review.googlesource.com/643950
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Original-Commit-Position: refs/heads/master@{#19612}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 774115c8c666c31ccf5f5a6da73ebba5b34d9ccf
2 files changed