Recently, a change was made to desktop/window capture on CrOS (and the same code is also used for Browser Window capture on Win/Linux) to fix a "freezing" problem.  https://codereview.chromium.org/1922143004/  This solution is a hack, in that it is mis-using a compositor animation observer callback to achieve guaranteed frame captures.  This solution is also very poor for performance across-the-board, whether you're considering CPU/GPU/Power usage or quality/smoothness of animating content.

What we need is something analogous to FrameSubscriber, the event-driven mechanism between DelegatedFrameHost and WebContentsVideoCaptureDevice used by tab capture.  Reference: https://code.google.com/p/chromium/codesearch#chromium/src/content/public/browser/render_widget_host_view_frame_subscriber.h&rcl=1461755891&l=30

Here's what I was thinking this might look like:

  // ui::Compositor
  class Compositor {
   public:
    // Clients implement this interface in order to receive notifications about
    // damage to a target (e.g., a ui::Layer) that is about to be resolved by
    // draw/commit.
    //
    // Example usage: When copying textures for screen capture, a client might
    // conditionally add a ui::CopyOutputRequest to a layer, according to its
    // configured sampling rate and current environmental conditions.
    class UpdateSubscriber {
     public:
      // Called when a damaged target will be composited. |damage_rect| is the
      // region within the target that it dirty and |present_time| is when the
      // result will be presented on the display.
      virtual bool WillCommitUpdate(const gfx::Rect& damage_rect,
                                    base::TimeTicks present_time) = 0;
    };

    ...

    void AddUpdateSubscriberForLayer(ui::Layer* layer,
                                     UpdateSubscriber* subscriber);
    void RemoveUpdateSubscriberForLayer(ui::Layer* layer,
                                        UpdateSubscriber* subscriber);

    ...
  };

Then, example usage by content::AuraWindowCaptureMachine might be:

  class CaptureSubscriber : public ui::Compositor::UpdateSubscriber {
    void WillCommitUpdate(damage_rect, present_time) override {
      if (damage_rect.IsEmpty())
        return;

      scoped_refptr<media::VideoFrame> output_frame;
      base::Callback<...> done_cb;
      if (oracle_->ObserveEventAndDecideCapture(damage_rect, present_time,
                                                &output_frame, &done_cb)) {
        target_->RequestCopyOfOutput(
            cc::CopyOutputRequest::CreateRequest(base::Bind(
                &AuraWindowCaptureMachine::DidCopyOutput,
                machine_, output_frame, present_time, done_cb)));
      }
    }

   private:
    ui::Layer* target_;
    base::WeakPtr<AuraWindowCaptureMachine> machine_;
    scoped_refptr<media::ThreadSafeCaptureOracle> oracle_;
  }

  ...

  // In AuraWindowCaptureMachine::InternalStart():
  window_->GetHost()->compositor()->AddUpdateSubscriberForLayer(window_->layer());