Completed the functionalities of SrtpTransport.

webrtc/p2p/base/fakepackettransport.h

Index: webrtc/p2p/base/fakepackettransport.h
diff --git a/webrtc/p2p/base/fakepackettransport.h b/webrtc/p2p/base/fakepackettransport.h
index 16af9a42aec5a5241e38b223cbd8dba7982e277a..82d39c23646712bc4a6ccf88ff8a19221923fe5f 100644
--- a/webrtc/p2p/base/fakepackettransport.h
+++ b/webrtc/p2p/base/fakepackettransport.h
@@ -86,6 +86,8 @@ class FakePacketTransport : public PacketTransportInternal {
   bool GetOption(Socket::Option opt, int* value) override { return true; }
   int GetError() override { return 0; }
 
+  const CopyOnWriteBuffer* last_sent_packet() { return &last_sent_packet_; }
+
  private:
   void set_writable(bool writable) {
     if (writable_ == writable) {
@@ -107,12 +109,14 @@ class FakePacketTransport : public PacketTransportInternal {
   }
 
   void SendPacketInternal(const CopyOnWriteBuffer& packet) {
+    last_sent_packet_ = packet;
     if (dest_) {
       dest_->SignalReadPacket(dest_, packet.data<char>(), packet.size(),
                               CreatePacketTime(0), 0);
     }
   }
 
+  CopyOnWriteBuffer last_sent_packet_;
   AsyncInvoker invoker_;
   std::string debug_name_;
   FakePacketTransport* dest_ = nullptr;