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}

webrtc/modules/rtp_rtcp/source/rtcp_receiver_unittest.cc

@@ -39,7 +39,9 @@ class TestTransport : public Transport,
   void SetRTCPReceiver(RTCPReceiver* rtcp_receiver) {
     rtcp_receiver_ = rtcp_receiver;
   }
-  bool SendRtp(const uint8_t* /*data*/, size_t /*len*/) override {
+  bool SendRtp(const uint8_t* /*data*/,
+               size_t /*len*/,
+               const PacketOptions& options) override {
     ADD_FAILURE();  // FAIL() gives a compile error.
     return false;
   }