Called before a download begins in response to a user-initiated action
(e.g. alt + link click or link click that returns a `Content-Disposition:
attachment` response from the server). |url| is the target download URL
and |request_function| is the target function (GET, POST, etc) nothrow. Return
true (1) to proceed with the download or false (0) to cancel the download.
Called before a download begins in response to a user-initiated action (e.g. alt + link click or link click that returns a `Content-Disposition: attachment` response from the server). |url| is the target download URL and |request_function| is the target function (GET, POST, etc) nothrow. Return true (1) to proceed with the download or false (0) to cancel the download.