[Cast] In Audio/VideoSender, drop frames when too-long a duration is in-flight.

media/cast/sender/rtp_timestamp_helper.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_CAST_SENDER_RTP_TIMESTAMP_HELPER_H_
 #define MEDIA_CAST_SENDER_RTP_TIMESTAMP_HELPER_H_
 
 #include "base/basictypes.h"
 #include "base/time/time.h"
 
 namespace media {
 namespace cast {
 
 // A helper class used to convert current time ticks into RTP timestamp.
 class RtpTimestampHelper {
  public:
   explicit RtpTimestampHelper(int frequency);
   ~RtpTimestampHelper();
 
-  // Compute a RTP timestamp using current time, last encoded time and
-  // last encoded RTP timestamp.
-  // Return true if |rtp_timestamp| is computed.
-  bool GetCurrentTimeAsRtpTimestamp(const base::TimeTicks& now,
-                                    uint32* rtp_timestamp) const;
+  // Compute a RTP timestamp from a TimeTicks, or a TimeTicks from a RTP
+  // timestamp; based on the currently stored capture time and its corresponding
+  // RTP timestamp.  Return true if |rtp_timestamp| is computed.
+  //
+  // The values are estimates because, ultimately, it's the encoders that decide
+  // what the RTP timestamps for each frame are going to be.  For example, in
+  // the case of audio, the TimeTicks may never be precise to an exact frame
+  // boundary; and so the computation here will result in a |rtp_timestamp|
+  // value that is near, but never exact to one emitted by the encoder.
+  bool EstimateRtpTimestamp(const base::TimeTicks& t,
+                            uint32* rtp_timestamp) const;
+  bool EstimateTimeTicks(uint32 rtp_timestamp, base::TimeTicks* t) const;
 
   // Store the capture time and the corresponding RTP timestamp for the
   // last encoded frame.
   void StoreLatestTime(base::TimeTicks capture_time, uint32 rtp_timestamp);
 
  private:
   int frequency_;
   base::TimeTicks last_capture_time_;
   uint32 last_rtp_timestamp_;
 
   DISALLOW_COPY_AND_ASSIGN(RtpTimestampHelper);
 };
 
 }  // namespace cast
 }  // namespace media
 
 #endif  // MEDIA_CAST_SENDER_RTP_TIMESTAMP_HELPER_H_