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This is a preliminary step to Onion souping content::PeerConnectionTracker.

The CL migrates both content::PeerConnectionTracker (renderer) and
content::PeerConnectionTrackerHost (browser) classes away from old
IPC mechanism, to use Mojo.

Additionally, the CL removes from:

- PeerConnectionTracker
  * the inheritance from content::RenderThreadObserver, as well as the respective
  overridden methods, namely OnControlMessageReceived().

- PeerConnectionTrackerHost
  * the inheritance from content::BrowserMessageFilter and
    content::BrowserAssociatedInterface, as well as the respective overridden,
    namely OnMessageReceived(), OverrideThreadForMessage(), OnChannelConnected(),
    OnChannelClosing().

It is important to note that PeerConnectionTrackerHost now operates solely
on the UI thread, so the CL removes some threading jumps between UI and IO
threads.

Last, content/common/media/peer_connection_tracker_messages.h (which declares
the macro-based IPC messages) is now gone.

BUG=787254
TBR=jam@chromium.org

Change-Id: I3ce5af1853384146d893da2a34c333aad80f5255
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1831197
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707672}