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/codecs/test/videoprocessor.h

@@ -250,6 +250,11 @@ class VideoProcessorImpl : public VideoProcessor {
       RTC_NOTREACHED();
       return -1;
     }
+    void Decoded(VideoFrame& frame,
+                 rtc::Optional<int32_t> decode_time_ms,
+                 rtc::Optional<uint8_t> qp) override {
+      RTC_NOTREACHED();
+    }
 
    private:
     VideoProcessorImpl* video_processor_;