add csp lv2 violation event

[gcbw]

https://www.w3.org/TR/CSP2/#securitypolicyviolationevent

[Schweinepriester]

+1

https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/63

The SecurityPolicyViolationEvent interface is now supported. It allows sending events when the Content-Security-Policy is violated (bug 1472661).