Set fetch request initiator when fetching a web app manifest

[benfrancis]

In '7.1 Obtaining a Manifest' in the W3C Web App Manifest specification it says:

8. Set request's initiator to "manifest".

The WHATWG specification for Fetch says initiator "is primarily a specification device to assist defining CSP and Mixed Content. It is not exposed to JavaScript."

So far I have not found a way to set this when using executeJavaScript() on an Electron <webview>.