Add a property `openDownloadsListOnStart` to the download object
prototype. It is true by default. When passed a false value, it will
prevent the usual behavior of opening the downloads panel when the
download starts. This will allow some (forthcoming) internal code in the
WebExtensions API to prevent showing the downloads panel when a download
is created without any user input (see bug 1759231 for details).
Differential Revision: https://phabricator.services.mozilla.com/D142503