Add encoded_data_length argument to didDownloadData delegate method

When leveraging ResourceRequest downloadToFile option to download
a File directly for XHR response, didDownloadData should propagate
encoded_data_length to notify it to the inspector.

When the Chromium-side patch is landed, didDownloadData definition
without encoded_data_length in WebURLLoaderClient will be removed.

BUG=269055

Review URL: https://chromiumcodereview.appspot.com/23632004

git-svn-id: svn://svn.chromium.org/blink/trunk@156838 bbb929c8-8fbe-4397-9dbb-9b2b20218538