Remove the dependency of TransportChannel and TransportChannelImpl.

DtlsTransportChannelWrapper is renamed to be DtlsTransport which inherits from
DtlsTransportInternal. There will be no concept of "channel" in p2p level.
Both P2PTransportChannel and DtlsTransport don't depend on TransportChannel
and TransportChannelImpl any more and they are removed in this CL.

BUG=none

Review-Url: https://codereview.webrtc.org/2606123002
Cr-Commit-Position: refs/heads/master@{#16173}
diff --git a/webrtc/p2p/base/dtlstransportinternal.h b/webrtc/p2p/base/dtlstransportinternal.h
index 780c128e..74fdd6d 100644
--- a/webrtc/p2p/base/dtlstransportinternal.h
+++ b/webrtc/p2p/base/dtlstransportinternal.h
@@ -22,6 +22,12 @@
 
 namespace cricket {
 
+enum PacketFlags {
+  PF_NORMAL = 0x00,       // A normal packet.
+  PF_SRTP_BYPASS = 0x01,  // An encrypted SRTP packet; bypass any additional
+                          // crypto provided by the transport (e.g. DTLS)
+};
+
 // DtlsTransportInternal is an internal interface that does DTLS.
 // Once the public interface is supported,
 // (https://www.w3.org/TR/webrtc/#rtcdtlstransport-interface)
@@ -94,6 +100,9 @@
     return transport_name() + " " + std::to_string(component());
   }
 
+ protected:
+  DtlsTransportInternal() {}
+
  private:
   RTC_DISALLOW_COPY_AND_ASSIGN(DtlsTransportInternal);
 };