This changes the ATK code to use AccessibleOrProxy::ChildAtPoint, since that already handles walking into OuterDocAccessibles appropriately.
Previously, atk_component_ref_accessible_at_point didn't work at all on OuterDocAccessibles.
Also, for ProxyAccessibles, we no longer adjust the coordinates for ATK_XY_WINDOW in the content process, as this depends on stuff that doesn't exist cross-platform and thus can't be used with AccessibleOrProxy.
Instead, we now handle this in the parent process before making the IPC call.
Differential Revision: https://phabricator.services.mozilla.com/D67986
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extra : moz-landing-system : lando
