Bug 1860257 - Enable iframe lazy loading inside WPT idlharness meta r=dom-core,hsivonen

The pref is nightly only now, and was not added here when switched
initially, causing future beta failures.

Differential Revision: https://phabricator.services.mozilla.com/D191631

testing/web-platform/meta/html/dom/idlharness.https.html.ini

@@ -1,4 +1,4 @@
-prefs: [dom.security.featurePolicy.experimental.enabled:true, dom.security.featurePolicy.header.enabled:true, dom.security.featurePolicy.webidl.enabled:true]
+prefs: [dom.security.featurePolicy.experimental.enabled:true, dom.security.featurePolicy.header.enabled:true, dom.security.featurePolicy.webidl.enabled:true, dom.iframe_lazy_loading.enabled:true]
 [idlharness.https.html?exclude=(Document|Window|HTML.*)]
   [AudioTrack interface: existence and properties of interface prototype object]
     expected: FAIL