Add stats for average QP per frame for VP8 (for received video streams).

webrtc/modules/video_coding/utility/include/vp8_header_parser.h

 /*
  *  Copyright (c) 2015 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 48 matching lines @@
   129, 131, 133, 135, 137, 139, 141, 143,
   145, 147, 149, 151, 153, 155, 157, 159,
   161, 163, 165, 167, 169, 171, 173, 175,
   177, 179, 181, 183, 185, 187, 189, 191,
   193, 195, 197, 199, 201, 203, 205, 207,
   209, 211, 213, 215, 217, 219, 221, 223,
   225, 227, 229, 231, 233, 235, 237, 239,
   241, 243, 245, 247, 249, 251, 253, 127
 };
 
-int GetQP(uint8_t* buf);
+// Gets the QP, QP range: [0, 127].
+// Returns true on success, false otherwise.
+bool GetQp(const uint8_t* buf, size_t length, int* qp);
 
 }  // namespace vp8
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_CODING_UTILITY_VP8_PARSE_HEADER_H_