In chrome://accessibility we can get the accessibility tree of chrome's native UI. This is implemented by calling RecursiveDumpAXPlatformNodeAsString in chrome/browser/accessibility/accessibility_ui.cc.

Because Chrome OS doesn't use AXPlatformNode for its native UI, RequestNativeUITree in the AccessibilityUI doesn't work. We can implement a special case for Chrome OS.

In the long term, we'd like to refactor native accessibility views, then this could be implemented more similarly across all platforms.