When a page opens a popup, the child tab shares the parent's
WebContent process via initialize_client_as_child(). Both tabs
register as views on the same WebContentClient.
If the child tab then does a cross-site navigation,
create_new_process_for_cross_site_navigation() would call
unregister_view() (correct) and then unconditionally send
CloseServer (wrong). This killed the WebContent process even
though the parent tab was still using it.
The unconditional async_close_server() predates the shared-process
popup model. It is no longer needed since unregister_view() already
sends CloseServer when the last view is removed.
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