Reland "[DataChannel] Send and receive packets on the network thread."

This reverts commit 7f16fcda0fd5bb625584b71311dd37b54c096136.

Reason for reland: Re-landing after addressing issues in downstream
code and hardening the ObserverAdapter from situations where attempted
usage of data channel proxies could occur after shutting down the
peer connection and terminating the network thread.

Original change's description:
> Revert "[DataChannel] Send and receive packets on the network thread."
>
> This reverts commit fe53fec24e02d2d644220f913c3f9ae596bbb2d9.
>
> Reason for revert: Speculative revert, may be breaking downstream project
>
> Original change's description:
> > [DataChannel] Send and receive packets on the network thread.
> >
> > This updates sctp channels, including work that happens between the
> > data channel controller and the transport, to run on the network
> > thread. Previously all network traffic related to data channels was
> > routed through the signaling thread before going to either the network
> > thread or the caller's thread (e.g. js thread in chrome). Now the
> > calls can go straight from the network thread to the JS thread with
> > enabling a special flag on the observer (see below) and similarly
> > calls to send data, involve 2 threads instead of 3.
> >
> > * Custom data channel observer adapter implementation that
> >   maintains compatibility with existing observer implementations in
> >   that notifications are delivered on the signaling thread.
> >   The adapter can be explicitly disabled for implementations that
> >   want to optimize the callback path and promise to not block the
> >   network thread.
> > * Remove the signaling thread copy of data channels in the controller.
> > * Remove several PostTask operations that were needed to keep things
> >   in sync (but the need has gone away).
> > * Update tests for the controller to consistently call
> >   TeardownDataChannelTransport_n to match with production.
> > * Update stats collectors (current and legacy) to fetch the data
> >   channel stats on the network thread where they're maintained.
> > * Remove the AsyncChannelCloseTeardown test since the async teardown
> >   step has gone away.
> > * Remove `sid_s` in the channel code since we only need the network
> >   state now.
> > * For the custom observer support (with and without data adapter) and
> >   maintain compatibility with existing implementations, added a new
> >   proxy macro that allows an implementation to selectively provide
> >   its own implementation without being proxied. This is used for
> >   registering/unregistering a data channel observer.
> > * Update the data channel proxy to map most methods to the network
> >   thread, avoiding the interim jump to the signaling thread.
> > * Update a plethora of thread checkers from signaling to network.
> >
> > Bug: webrtc:11547
> > Change-Id: Ib4cff1482e31c46008e187189a79e967389bc518
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/299142
> > Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> > Reviewed-by: Henrik Boström <hbos@webrtc.org>
> > Cr-Commit-Position: refs/heads/main@{#39760}
>
> Bug: webrtc:11547
> Change-Id: Id0d65594bf727ccea5c49093c942b09714d101ad
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/300341
> Auto-Submit: Andrey Logvin <landrey@webrtc.org>
> Owners-Override: Andrey Logvin <landrey@webrtc.org>
> Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#39764}

Bug: webrtc:11547
Change-Id: I47dfa7e7168be0cd2faab4f8f3ebf110c3728af5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/300360
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39786}
diff --git a/pc/peer_connection.cc b/pc/peer_connection.cc
index fdbd32b..82c5914 100644
--- a/pc/peer_connection.cc
+++ b/pc/peer_connection.cc
@@ -2286,7 +2286,7 @@
 }
 
 std::vector<DataChannelStats> PeerConnection::GetDataChannelStats() const {
-  RTC_DCHECK_RUN_ON(signaling_thread());
+  RTC_DCHECK_RUN_ON(network_thread());
   return data_channel_controller_.GetDataChannelStats();
 }