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/test/testAPI/test_api.cc

@@ -31,7 +31,9 @@ void LoopBackTransport::DropEveryNthPacket(int n) {
   packet_loss_ = n;
 }
 
-bool LoopBackTransport::SendRtp(const uint8_t* data, size_t len) {
+bool LoopBackTransport::SendRtp(const uint8_t* data,
+                                size_t len,
+                                const PacketOptions& options) {
   count_++;
   if (packet_loss_ > 0) {
     if ((count_ % packet_loss_) == 0) {