Bug 1643798 - Fix feature policy check for fullscreen so that it gets reflected properly in document.fullscreenEnabled. r=baku

See bug 1606660 comment 8 as to why checking it only in
Element.requestFullscreen is wrong.

Do you know how to test this? I'm not very familiar with feature-policy.

Differential Revision: https://phabricator.services.mozilla.com/D78567

dom/base/Element.cpp

@@ -3100,10 +3100,8 @@ static const char* GetFullscreenError(CallerType aCallerType,
     return nullptr;
   }
 
-  // Ensure feature policy allows using the fullscreen API
-  if (!FeaturePolicyUtils::IsFeatureAllowed(aDocument,
-                                            NS_LITERAL_STRING("fullscreen"))) {
-    return "FullscreenDeniedFeaturePolicy";
+  if (const char* error = aDocument->GetFullscreenError(aCallerType)) {
+    return error;
   }
 
   // Bypass user interaction checks if preference is set