Bug 1166910 - put referrer attribute behind pref in webidl, r=bz

--HG--
extra : rebase_source : 8ea13d4057a5b9f1362291e3be1ffc4342eab5af
extra : histedit_source : 5291f2ed808051c7f9042189130041a1e56bcd96

dom/webidl/HTMLImageElement.webidl

@@ -29,7 +29,7 @@ interface HTMLImageElement : HTMLElement {
            attribute DOMString? crossOrigin;
            [SetterThrows]
            attribute DOMString useMap;
-           [SetterThrows]
+           [SetterThrows, Pref="network.http.enablePerElementReferrer"]
            attribute DOMString referrer;
            [SetterThrows]
            attribute boolean isMap;