Called on the IO thread before a resource request is loaded. The |browser| and |frame| values represent the source of the request, and may be NULL for requests originating from service workers or cef_urlrequest_t. To redirect or change the resource load optionally modify |request|. Modification of the request URL will be treated as a redirect. Return RV_CONTINUE to continue the request immediately. Return RV_CONTINUE_ASYNC and call cef_callback_t functions at a later time to continue or cancel the request asynchronously. Return RV_CANCEL to cancel the request immediately.