From discussion with dtseng@ on https://chromium-review.googlesource.com/c/chromium/src/+/1162520

> > A side question: is the remote tree currently exposed on the extension as an AXTree or is it part of the desktop tree? If the former, I don't see a call to destroy the tree itself (should go through AutomationEventRouter::GetInstance()->DispatchTreeDestroyedEvent  in the Chrome process).
> 
> I think it's part of the desktop tree, though I'm not very familiar with the distinction. I set ax::mojom::IntAttribute::kChildTreeId on a views::View in the browser process that represents the remote app, see here:  
> 
> https://cs.chromium.org/chromium/src/ash/wm/non_client_frame_controller.cc?type=cs&sq=package:chromium&g=0&l=257
> 
> The code in this CL runs in the remote app process, not in the browser process, so it can't call directly into AutomationEventRouter. I have a browser-side proxy that speaks mojo to the remote app in AXHostService, see here:
> 
> https://chromium.googlesource.com/chromium/src/+/master/chrome/browser/chromeos/accessibility/ax_host_service.h
> 
> Do you think I should be manually doing DispatchTreeDestroyedEvent() when the remote app closes? Is there some way I can dump the AX extension's final tree state, so I can see what nodes it ended up with?
> 
> 

Yes, you should be dispatching tree destroyed.

The data flows from ash -> chrome -> extension.

The chrome process hooks up the chlid tree to the larger desktop tree. The desktop tree itself never gets destroyed, so you perhaps didn't get that bit when you modeled off of AutomationManagerAura/AXTreeSourceAura.

The extension process, where the data gets unserialized lives at:
chrome/renderer/extension/automation_internal_custom_bindings.cc. Here, we manage a collection of AXTree's from all processes, including your remote app.

You can see other sources of 
DispatchTreeDestroyedEvent
in
chrome/browser/extensions/api/automation_internal/automation_internal_api.cc
and
c/b/c/arc/ax_tree_source_arc.cc

The extension process deletes the target (unserialized) tree once this event/message gets received.