RenderWidgetHostLatencyTracker latency_tracker_ lives in RenderWidgetHostImpl.

When we receive a CompositorFrame in RenderWidgetHostImpl::SubmitCompositorFrame, we add to its LatencyInfo.

latency_tracker_.OnSwapCompositorFrame(&frame.metadata.latency_info);

This cannot work with Viz. 

Maybe we can add a privileged host interface to Viz (A LatencyInfoObserver) that observes latency infos in frames, and passes them along to the host?

This seems expensive. Maybe there's a better way to propagate this? Perhaps once per aggregation?