Remove ASSERT in TransportChannelProxy::SetImplementation, when proxy already set to same transport channel impl.
Remove ASSERT in TransportChannelProxy::SetImplementation, when
proxy already set to same transport channel impl.

Since session can call SetImplementation multiple times with or without BUNDLE, there are cases when SetImplementation is called with same impl (OnRemoteCandidates/PushdownTransportDescription/SetupMux). Also variables in
cricket::TransportProxy like |connecting_| and |negotiated_| are accessed
both between worker thread and signaling threads (which calls for bigger change
on how session interacts with Transport and TransportChannelProxy). I have a created a separate bug to address later issue.

Also if single thread used as worker and signaling thread, we can end up
calling SetLocalDescription and OnRemoteCandidates in same call sequence, which
will end up calling SetImplementation twice.

R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12019007

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 54fd70046d671d121a71a27181ea47ad12b27d48

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