Align interface for securitypolicyviolation with CSP spec

[dontcallmedom]

According to CSP, the interface for the securitypolicyviolation event is SecurityPolicyViolationEvent, not Event https://w3c.github.io/webappsec-csp/#report-violation

https://github.com/web-platform-tests/wpt/blob/master/content-security-policy/navigation/to-javascript-url-script-src.html and https://wpt.fyi/results/content-security-policy/navigation/to-javascript-url-script-src.html?label=experimental&label=master&aligned illustrates that's indeed what's implemented (at least in Chromium & Webkit)

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/indices.html ( diff )
/infrastructure.html ( diff )

[domenic]

I'm not 100% clear why we have this in the event index in the first place, or what the criteria are for being in the event index (is it just anything in GlobalEventHandlers??) but this does seem like a strict improvement in accuracy, so let's merge. Thanks!