Bug 1615234 - Make the lazyload pref actually be honored. r=hiro

Right now it only controls the visibility of HTMLImageElement.loading.

Differential Revision: https://phabricator.services.mozilla.com/D62723

dom/html/HTMLImageElement.cpp

@@ -1250,6 +1250,10 @@ void HTMLImageElement::SetLazyLoading() {
     return;
   }
 
+  if (!StaticPrefs::dom_image_lazy_loading_enabled()) {
+    return;
+  }
+
   // If scripting is disabled don't do lazy load.
   // https://whatpr.org/html/3752/images.html#updating-the-image-data
   if (!OwnerDoc()->IsScriptEnabled()) {