Reland of Add QP sum stats for received streams. (patchset #2 id:300001 of https://codereview.webrtc.org/2680893002/ )

Reason for revert:
Fix the problem.

Original issue's description:
> Revert of Add QP sum stats for received streams. (patchset #10 id:180001 of https://codereview.webrtc.org/2649133005/ )
>
> Reason for revert:
> Breaks downstream build.
>
> Original issue's description:
> > 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}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/ff0e72fd165facac27f0313aa178648782e63bc4
>
> TBR=hta@webrtc.org,hbos@webrtc.org,sprang@webrtc.org,magjed@webrtc.org,stefan@webrtc.org,sakal@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:6541
>
> Review-Url: https://codereview.webrtc.org/2680893002 .
> Cr-Commit-Position: refs/heads/master@{#16480}
> Committed: https://chromium.googlesource.com/external/webrtc/+/69fb2cca4d54f3df7ceddcd1c3e9b0ad80fa849b

TBR=hta@webrtc.org,hbos@webrtc.org,sprang@webrtc.org,magjed@webrtc.org,stefan@webrtc.org,skvlad@webrtc.org
BUG=webrtc:6541

Review-Url: https://codereview.webrtc.org/2681663005
Cr-Commit-Position: refs/heads/master@{#16511}
diff --git a/webrtc/modules/video_coding/generic_decoder.h b/webrtc/modules/video_coding/generic_decoder.h
index 2d0007b..48e2519 100644
--- a/webrtc/modules/video_coding/generic_decoder.h
+++ b/webrtc/modules/video_coding/generic_decoder.h
@@ -37,11 +37,13 @@
     void SetUserReceiveCallback(VCMReceiveCallback* receiveCallback);
     VCMReceiveCallback* UserReceiveCallback();
 
-    virtual int32_t Decoded(VideoFrame& decodedImage);  // NOLINT
-    virtual int32_t Decoded(VideoFrame& decodedImage,   // NOLINT
-                            int64_t decode_time_ms);
-    virtual int32_t ReceivedDecodedReferenceFrame(const uint64_t pictureId);
-    virtual int32_t ReceivedDecodedFrame(const uint64_t pictureId);
+    int32_t Decoded(VideoFrame& decodedImage) override;
+    int32_t Decoded(VideoFrame& decodedImage, int64_t decode_time_ms) override;
+    void Decoded(VideoFrame& decodedImage,
+                 rtc::Optional<int32_t> decode_time_ms,
+                 rtc::Optional<uint8_t> qp) override;
+    int32_t ReceivedDecodedReferenceFrame(const uint64_t pictureId) override;
+    int32_t ReceivedDecodedFrame(const uint64_t pictureId) override;
 
     uint64_t LastReceivedPictureID() const;
     void OnDecoderImplementationName(const char* implementation_name);