Labels derived from user-minted privileges are not parsed by CSP

[alejandrorusso]

User-minted privileges can be used to confine code to the browser by raising COWL.privacyLabel with the label derived from such privilege. At the moment, CSP is not parsing labels derived from user-minted privileges. More concretely,

var p = new FreshPrivilege(), 
COWL.privacyLabel = COWL.privacyLabel.and(p.asLabel) ; 

produces the error:

Content Security Policy: Failed to parse unrecognized source moz-nullprincipal:{c5ed1219-b1d0-450e-8d3d-c20ceb33ae96}