Bug 1839316: part 5) Guard the "fetchpriority" attribute behind a pref. r=kershaw,webidl,smaug,saschanaz

Allows landing the previous parts in order to avoid future
merge-conflicts.

Differential Revision: https://phabricator.services.mozilla.com/D185296

dom/webidl/HTMLScriptElement.webidl

@@ -30,7 +30,7 @@ interface HTMLScriptElement : HTMLElement {
   attribute DOMString referrerPolicy;
   [CEReactions, Throws]
   attribute DOMString text;
-  [CEReactions]
+  [Pref="network.fetchpriority.enabled", CEReactions]
   attribute DOMString fetchPriority;
 
   static boolean supports(DOMString type);

modules/libpref/init/StaticPrefList.yaml

@@ -11728,6 +11728,13 @@
   value: false
   mirror: always
 
+# Indicates whether the `fetchpriority` attribute for elements which support it
+# (e.g. `<script>`) is enabled.
+- name: network.fetchpriority.enabled
+  type: bool
+  value: false
+  mirror: always
+
 # Enables `<link rel="preload">` tag and `Link: rel=preload` response header
 # handling.
 - name: network.preload

testing/web-platform/meta/html/scripting/the-script-element/attr-script-fetchpriority.html.ini

@@ -0,0 +1,2 @@
+[attr-script-fetchpriority.html]
+  prefs: [network.fetchpriority.enabled:true]