Pass capture time (wallclock) to the RTP sender to compute transmission offset

- Change how the transmission offset is calculated, to
  incorporate the time since the frame was captured.
- Break out RtpRtcpClock and move it to system_wrappers.
- Use RtpRtcpClock to set the capture time in ms in the capture module.
  We must use the same clock as in the RTP module to be able to measure
  the time from capture until transmission.
- Enables the RTP header extension for packet transmission time offsets.

BUG=
TEST=trybots

Review URL: https://webrtc-codereview.appspot.com/666006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2489 4adac7df-926f-26a2-2b94-8c16560cd09d

src/modules/rtp_rtcp/source/rtp_rtcp_impl.cc

diff --git a/src/modules/rtp_rtcp/source/rtp_rtcp_impl.cc b/src/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
index b2851a7..56a4236 100644
--- a/src/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
+++ b/src/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
@@ -798,6 +798,7 @@
     FrameType frameType,
     WebRtc_Word8 payloadType,
     WebRtc_UWord32 timeStamp,
+    int64_t capture_time_ms,
     const WebRtc_UWord8* payloadData,
     WebRtc_UWord32 payloadSize,
     const RTPFragmentationHeader* fragmentation,
@@ -818,6 +819,7 @@
     return _rtpSender.SendOutgoingData(frameType,
                                        payloadType,
                                        timeStamp,
+                                       capture_time_ms,
                                        payloadData,
                                        payloadSize,
                                        fragmentation,
@@ -847,6 +849,7 @@
     return rtpSender.SendOutgoingData(frameType,
                                       payloadType,
                                       timeStamp,
+                                      capture_time_ms,
                                       payloadData,
                                       payloadSize,
                                       fragmentation,
@@ -863,6 +866,7 @@
       retVal = rtpSender.SendOutgoingData(frameType,
                                           payloadType,
                                           timeStamp,
+                                          capture_time_ms,
                                           payloadData,
                                           payloadSize,
                                           fragmentation,
@@ -878,6 +882,7 @@
       retVal = rtpSender.SendOutgoingData(frameType,
                                           payloadType,
                                           timeStamp,
+                                          capture_time_ms,
                                           payloadData,
                                           payloadSize,
                                           fragmentation,