Bug 1686234 - Add missing includes. r=peterv

Also add missing forward declaration for FeaturePolicy.

Differential Revision: https://phabricator.services.mozilla.com/D101466

dom/html/HTMLIFrameElement.h

@@ -8,12 +8,15 @@
 #define mozilla_dom_HTMLIFrameElement_h
 
 #include "mozilla/Attributes.h"
+#include "nsGenericHTMLElement.h"
 #include "nsGenericHTMLFrameElement.h"
 #include "nsDOMTokenList.h"
 
 namespace mozilla {
 namespace dom {
 
+class FeaturePolicy;
+
 class HTMLIFrameElement final : public nsGenericHTMLFrameElement {
  public:
   explicit HTMLIFrameElement(