We transport the aggregated HitTestRegionList to the viz host. Given the top level CompositorFrame is manufactured in FrameGenerator, we can trigger a new top level CompositorFrame every time the aggregated HitTestRegionList changes, walk the list, and generate corresponding DebugBorderDrawQuads that we overlay on top of the window manager's SurfaceDrawQuad in FrameGenerator. This can be a mode that we turn on and off.