Revert of Changed P2PTestConductor to use a separate WorkerThread. (patchset #1 id:1 of https://codereview.webrtc.org/1859933002/ )
Revert of Changed P2PTestConductor to use a separate WorkerThread. (patchset #1 id:1 of https://codereview.webrtc.org/1859933002/ )

Reason for revert:
Causes P2PTestConductor.LocalP2PTestDtlsTransferCaller to fail on Win dbg.

https://build.chromium.org/p/client.webrtc/builders/Win32%20Debug/builds/7469/steps/peerconnection_unittests/logs/stdio

e:\b\build\slave\win\build\src\webrtc\api\peerconnection_unittest.cc(1221): error: Value of: initiating_client_->ice_connection_state()
  Actual: 2
Expected: webrtc::PeerConnectionInterface::kIceConnectionCompleted
Which is: 3

Original issue's description:
> Changed P2PTestConductor to use a separate WorkerThread.
>
> P2PTestConductor currently use the current thread both as a signaling thread and a worker thread. Although convenient while debugging, it can also hide real bugs. An example is https://codereview.webrtc.org/1766653002/#ps420001 where the worker thread is deadlocked in the track proxy due to that the worker thread waits for the signaling thread but the proxy in turns invokes the worker thread..... That bug was only discovered on Android. I suggest we let the P2PTestConductor use a separate thread as a worker thread to better cover how PeerConnections are used in reality.
>
> BUG=webrtc:5426
>
> Committed: https://crrev.com/6172401972c54813698d73580779d675d99178b4
> Cr-Commit-Position: refs/heads/master@{#12252}

TBR=nisse@webrtc.org,pthatcher@webrtc.org,deadbeef@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5426

Review URL: https://codereview.webrtc.org/1866503003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#12255}

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