Bug 1632447 - Disable window.external/sidebar.AddSearchProvider by preference. r=baku

This makes them both operate as dummy functions.

Differential Revision: https://phabricator.services.mozilla.com/D76329

dom/webidl/Window.webidl

@@ -554,8 +554,9 @@ partial interface Window {
 
 #ifdef HAVE_SIDEBAR
 // Mozilla extension
+// Sidebar is deprecated and it will be removed in the next cycles. See bug 1640138.
 partial interface Window {
-  [Replaceable, Throws, UseCounter, Pref="dom.sidebar.enabled"]
+  [Replaceable, Throws, UseCounter]
   readonly attribute (External or WindowProxy) sidebar;
 };
 #endif

modules/libpref/init/StaticPrefList.yaml

@@ -2583,7 +2583,7 @@
 # cycles.
 - name: dom.sidebar.enabled
   type: bool
-  value: true
+  value: false
   mirror: always
 
 # Whether automatic storage access granting heuristics should be turned on.