Called just before a browser is destroyed. Release all references to the browser object and do not attempt to execute any functions on the browser object (other than IsValid, GetIdentifier or IsSame) after this callback returns. cef_frame_handler_t callbacks related to final main frame destruction will arrive after this callback and cef_browser_t::IsValid will return false (0) at that time. Any in-progress network requests associated with |browser| will be aborted when the browser is destroyed, and cef_resource_request_handler_t callbacks related to those requests may still arrive on the IO thread after this callback. See cef_frame_handler_t and do_close() documentation for additional usage information.