Add a new finch experiment to control the system buffer size.

content/browser/renderer_host/p2p/socket_host_udp.h

Index: content/browser/renderer_host/p2p/socket_host_udp.h
diff --git a/content/browser/renderer_host/p2p/socket_host_udp.h b/content/browser/renderer_host/p2p/socket_host_udp.h
index 6db6faa0e929d5c22ab895c2f82765d287ac7528..4a5bd434b207c1a0d92330c14750f677cd4e3d7c 100644
--- a/content/browser/renderer_host/p2p/socket_host_udp.h
+++ b/content/browser/renderer_host/p2p/socket_host_udp.h
@@ -43,6 +43,20 @@ class CONTENT_EXPORT P2PSocketHostUdp : public P2PSocketHost {
       int id) override;
   bool SetOption(P2PSocketOption option, int value) override;
 
+  // Helper functions to hijack upper 32 bits for latency calculation.
+  // PackCallRecord will return a "CallRecord".  A "CallRecord" has its lower 32
+  // bits as the lower 32 bits of packet id (without the random prefix) and its
+  // higher 32 bits as the lower 32 bits of tick count when the packet arrives
+  // at Send().
+  static uint64 PackCallRecord(const uint64 packet_id, const uint64 ticks_now);

[Alexei Svitkine (slow), 2014/11/04 15:17:50]

Nit: Usually primitive params aren't marked const.

[guoweis2, 2014/11/07 22:18:19]

Code removed.

+  // |ticks_diff| contains the difference between the time when
+  // |packed_call_record| is packed and |ticks_now|.
+  static void UnpackCallRecord(const uint64 packed_call_record,
+                               const uint64 random_socket_id_prefix,
+                               const uint64 ticks_now,
+                               uint64* packet_id,
+                               uint64* ticks_diff);
+
  private:
   friend class P2PSocketHostUdpTest;
 
@@ -63,6 +77,8 @@ class CONTENT_EXPORT P2PSocketHostUdp : public P2PSocketHost {
 
   void OnError();
 
+  void SetSendBufferSize();
+
   void DoRead();
   void OnRecv(int result);
   void HandleReadResult(int result);
@@ -79,6 +95,18 @@ class CONTENT_EXPORT P2PSocketHostUdp : public P2PSocketHost {
   bool send_pending_;
   net::DiffServCodePoint last_dscp_;
 
+  // When the renderer sends the packet through IPC, an unique packet_id is
+  // stamped at each packet to allow tracing infrastructure to uniquely identify
+  // each packet. The packet id is always constructed as a random number at
+  // upper 32 bits and a sequence number starting from 0 at lower 32 bits. The
+  // packet id is also sent to socket::SendTo as a callback parameter in case
+  // when the socket can't send it synchronously. The callback only allows an
+  // int64 as a parameter. To allow us to track the latency across the
+  // asynchronous socket::SendTo, we are hijacking the higher 32 bits for
+  // latency calculation. The |random_socket_id_prefix| is kept here so we could
+  // reconstruct the correct packet id.
+  uint64 random_socket_id_prefix_;

[Alexei Svitkine (slow), 2014/11/04 15:17:50]

Nit: I think for new code, uint64_t is preferred (i.e. types with _t suffix).

[guoweis2, 2014/11/07 22:18:19]

Code removed.

+
   // Set of peer for which we have received STUN binding request or
   // response or relay allocation request or response.
   ConnectedPeerSet connected_peers_;