Add QP sum stats for received streams.

This is not implemented yet in any of the decoders.

BUG=webrtc:6541

Review-Url: https://codereview.webrtc.org/2649133005
Cr-Commit-Position: refs/heads/master@{#16475}

webrtc/modules/video_coding/include/mock/mock_video_codec_interface.h

@@ -57,6 +57,10 @@ class MockDecodedImageCallback : public DecodedImageCallback {
   MOCK_METHOD2(Decoded,
                int32_t(VideoFrame& decodedImage,  // NOLINT
                        int64_t decode_time_ms));
+  MOCK_METHOD3(Decoded,
+               void(VideoFrame& decodedImage,  // NOLINT
+                    rtc::Optional<int32_t> decode_time_ms,
+                    rtc::Optional<uint8_t> qp));
   MOCK_METHOD1(ReceivedDecodedReferenceFrame,
                int32_t(const uint64_t pictureId));
   MOCK_METHOD1(ReceivedDecodedFrame, int32_t(const uint64_t pictureId));