Fix DCHECK in the chromoting host when disconnecting a WebRTC connection.

WebRTC creates threads internally. Thread::Stop() is a blocking operation,
so it's not allowed on the network thread. To work around this problem
WebrtcTransport::PeerConnectionWrapper uses ScopedAllowIO when closing
PeerConnection. But it's not longer enough - some threads are stopped
later when the factory is destroyed. Updated PeerConnectionWrapper
destructor to destroy the PeerConnectionFactory with ScopedAllowIO.

BUG=714699

Review-Url: https://codereview.chromium.org/2847803003
Cr-Commit-Position: refs/heads/master@{#467838}

remoting/protocol/webrtc_transport.cc

@@ -208,6 +208,9 @@ class WebrtcTransport::PeerConnectionWrapper
     // See crbug.com/660081.
     base::ThreadRestrictions::ScopedAllowIO allow_io;
     peer_connection_->Close();
+    peer_connection_ = nullptr;
+    peer_connection_factory_ = nullptr;
+    audio_module_ = nullptr;
   }
 
   WebrtcAudioModule* audio_module() {