What metadata do you need exactly?

Natural size, for example, is part of the metadata, but is also reported via. OnVideoNaturalSizeChanged(). Perhaps we should just be hooking up callbacks for each of the additional things that you need.

Currently we need to know the change on natural size and codec. But we may need more (or add more) in Audio/VideoDecoderConfig, for example, fps, profile, to check whether it matches receiver's capability. 

I do see some of the config changes are reported via OnVideoNatrualSizeChanged() and OnVideoOpacityChange() by Renderer. Since either the natural size or the video opacity can only be changed when config changes, can we just call OnMetaData() to report the new config to WMPI when config config changes (instead checking in VideoRendererImpl::CheckForMetadataChanges())? Currently the natural size is reported both through OnMetaData() and OnVideoNaturalSizeChanged() in WMPI.

It seems that both Renderer and DemuxerStream can know when config changes. Is there any preference on who should report the change? 

CL is out for review: https://codereview.chromium.org/2620433002/, which directly inform MediaObserver with updated meta data when OnVideoNaturalSizeChanged(). 

Another issue is to estimate the frame rate and report changes. 
For non-mse sources, the "r_frame_rate" field in AVStream is the estimated frame rate (https://cs.chromium.org/chromium/src/third_party/ffmpeg/libavformat/avformat.h?rcl=0&l=1094). But for mse sources, no frame rate is estimated. 
I tried estimate it in VideoRendererImpl::FrameReady() using the following logic:
fps = 1000.0 / (frame->timestamp()-last_frame_ready_).InMillisecondsF();
It seems work pretty well for both local sources and mse sources (YouTube, Vimeo). The estimate is also stable (almost constant). 
And then we can report any changes by adding a callback (similar to that for the natural size change). sg?

I don't expect this computation to be very stable; in particular blackness is often encoded with extra long frames; you probably want the maximum frame rate over a window.

What exactly are you using the frame rate for? Perhaps the requirements you have are different from what I imagine.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/516ef6d9c0c19352fb80ecaa4e635e60501125fb

commit 516ef6d9c0c19352fb80ecaa4e635e60501125fb
Author: xjz <xjz@chromium.org>
Date: Sat Jan 07 00:23:06 2017

Inform MediaObserver when video natural size changes.

BUG= 678339 

Review-Url: https://codereview.chromium.org/2620433002
Cr-Commit-Position: refs/heads/master@{#442107}

[modify] https://crrev.com/516ef6d9c0c19352fb80ecaa4e635e60501125fb/media/blink/webmediaplayer_impl.cc