Revert of Implement RTCConfiguration.iceCandidatePoolSize. (patchset #7 id:120001 of https://codereview.webrtc.org/1956453003/ )

Reason for revert:
Breaks remoting_unittests. They defined their own operator== which conflicts with this one.

I'll remove the operator== in a roll CL. But until it's approved, I'm reverting this so the FYI bots will pass.

Original issue's description:
> Implement RTCConfiguration.iceCandidatePoolSize.
>
> It works by creating pooled PortAllocatorSessions which can be picked up
> by a P2PTransportChannel when needed (after a local description is set).
>
> This can optimize candidate gathering time when there is some time between
> creating a PeerConnection and setting a local description.
>
> R=pthatcher@webrtc.org
>
> Committed: https://chromium.googlesource.com/external/webrtc/+/48e9d05f510b1616c81303944008f75825971802

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

Review-Url: https://codereview.webrtc.org/1972043004
Cr-Commit-Position: refs/heads/master@{#12709}
diff --git a/webrtc/api/peerconnection.h b/webrtc/api/peerconnection.h
index 15ecc3f..862c6fb 100644
--- a/webrtc/api/peerconnection.h
+++ b/webrtc/api/peerconnection.h
@@ -131,7 +131,7 @@
   void SetRemoteDescription(SetSessionDescriptionObserver* observer,
                             SessionDescriptionInterface* desc) override;
   bool SetConfiguration(
-      const PeerConnectionInterface::RTCConfiguration& configuration) override;
+      const PeerConnectionInterface::RTCConfiguration& config) override;
   bool AddIceCandidate(const IceCandidateInterface* candidate) override;
   bool RemoveIceCandidates(
       const std::vector<cricket::Candidate>& candidates) override;
@@ -210,8 +210,6 @@
     return factory_->signaling_thread();
   }
 
-  rtc::Thread* worker_thread() const { return factory_->worker_thread(); }
-
   void PostSetSessionDescriptionFailure(SetSessionDescriptionObserver* observer,
                                         const std::string& error);
   void PostCreateSessionDescriptionFailure(
@@ -353,12 +351,6 @@
   // or nullptr if not found.
   DataChannel* FindDataChannelBySid(int sid) const;
 
-  // Called when first configuring the port allocator.
-  bool InitializePortAllocator_w(const RTCConfiguration& configuration);
-  // Called when SetConfiguration is called. Only a subset of the configuration
-  // is applied.
-  bool ReconfigurePortAllocator_w(const RTCConfiguration& configuration);
-
   // Storing the factory as a scoped reference pointer ensures that the memory
   // in the PeerConnectionFactoryImpl remains available as long as the
   // PeerConnection is running. It is passed to PeerConnection as a raw pointer.
@@ -405,7 +397,11 @@
   std::vector<rtc::scoped_refptr<RtpSenderInterface>> senders_;
   std::vector<rtc::scoped_refptr<RtpReceiverInterface>> receivers_;
 
+  // The session_ unique_ptr is declared at the bottom of PeerConnection
+  // because its destruction fires signals (such as VoiceChannelDestroyed)
+  // which will trigger some final actions in PeerConnection...
   std::unique_ptr<WebRtcSession> session_;
+  // ... But stats_ depends on session_ so it should be destroyed even earlier.
   std::unique_ptr<StatsCollector> stats_;
 };