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

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

Index: webrtc/modules/video_coding/utility/include/vp8_header_parser.h
diff --git a/webrtc/modules/video_coding/utility/include/vp8_header_parser.h b/webrtc/modules/video_coding/utility/include/vp8_header_parser.h
index 839e0933fc28c10bbbded646503c5db39a7ade9e..88796ecd0e7639e88ac5a4b3dbab6a5b8d7dd823 100644
--- a/webrtc/modules/video_coding/utility/include/vp8_header_parser.h
+++ b/webrtc/modules/video_coding/utility/include/vp8_header_parser.h
@@ -66,7 +66,9 @@ const uint8_t kVP8NewRange[128] = {
   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