Wire up pacer-based padding.

This connects the pacer-based padding with the RTP modules, which will
generate padding packets roughly according to what the pacer suggests.
It will only generate padding packets of maximum size to keep the number
off padding packets as small as possible. This also sets a limit of how much
padding + media bitrate which the pacer is allowed to "request" from the
RTP modules.

Padding will for now only be generated by the first sending RTP module.

BUG=1837
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1612005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4234 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -190,6 +190,9 @@ class ModuleRtpRtcpImpl : public RtpRtcp {
 
   virtual void TimeToSendPacket(uint32_t ssrc, uint16_t sequence_number,
                                 int64_t capture_time_ms);
+  // Returns the number of padding bytes actually sent, which can be more or
+  // less than |bytes|.
+  virtual int TimeToSendPadding(int bytes);
   // RTCP part.
 
   // Get RTCP status.