Update RateStatistics to handle too-little-data case.

webrtc/common_video/include/bitrate_adjuster.h

 /*
  *  Copyright 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 29 matching lines @@
 
   // Sets the desired bitrate in bps (bits per second).
   // Should be called at least once before Update.
   void SetTargetBitrateBps(uint32_t bitrate_bps);
   uint32_t GetTargetBitrateBps() const;
 
   // Returns the adjusted bitrate in bps.
   uint32_t GetAdjustedBitrateBps() const;
 
   // Returns what we think the current bitrate is.
-  uint32_t GetEstimatedBitrateBps();
+  rtc::Optional<uint32_t> GetEstimatedBitrateBps();
 
   // This should be called after each frame is encoded. The timestamp at which
   // it is called is used to estimate the output bitrate of the encoder.
   // Should be called from only one thread.
   void Update(size_t frame_size);
 
  private:
   // Returns true if the bitrate is within kBitrateTolerancePct of bitrate_bps.
   bool IsWithinTolerance(uint32_t bitrate_bps, uint32_t target_bitrate_bps);
 
@@ skipping 19 matching lines @@
   RateStatistics bitrate_tracker_ GUARDED_BY(crit_);
   // The last time we tried to adjust the bitrate.
   uint32_t last_bitrate_update_time_ms_ GUARDED_BY(crit_);
   // The number of frames since the last time we tried to adjust the bitrate.
   uint32_t frames_since_last_update_ GUARDED_BY(crit_);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_COMMON_VIDEO_INCLUDE_BITRATE_ADJUSTER_H_