Add a PacketOptions struct to webrtc::Transport.

This allows us to pass packet meta data, such as transport sequence
number, to libjingle and further down to the socket implementation. A
similar struct already exist in libjingle, see rtc::PacketOptions in asyncpacketsocket.h.

BUG=4173

Review URL: https://codereview.webrtc.org/1376673004

Cr-Commit-Position: refs/heads/master@{#10144}
diff --git a/webrtc/test/direct_transport.h b/webrtc/test/direct_transport.h
index a0aaecb..1c6f937 100644
--- a/webrtc/test/direct_transport.h
+++ b/webrtc/test/direct_transport.h
@@ -39,7 +39,9 @@
   virtual void StopSending();
   virtual void SetReceiver(PacketReceiver* receiver);
 
-  bool SendRtp(const uint8_t* data, size_t length) override;
+  bool SendRtp(const uint8_t* data,
+               size_t length,
+               const PacketOptions& options) override;
   bool SendRtcp(const uint8_t* data, size_t length) override;
 
  private: