Implement Subresource Integrity

Implemented response validation part of
https://w3c.github.io/webappsec-subresource-integrity/.
Implemented step eighteen of the main fetch. If a request has integrity
metadata, then following steps are performed
*Wait for response body
*If the response does not have a termination reason and response does not
match request’s integrity metadata, set response to a
network error.# Please enter the commit message for your changes. Lines starting

components/script/dom/webidls/HTMLScriptElement.webidl

@@ -12,6 +12,7 @@ interface HTMLScriptElement : HTMLElement {
            attribute DOMString? crossOrigin;
            [Pure]
            attribute DOMString text;
+           attribute DOMString integrity;
 
   // also has obsolete members
 };