Bug 1700963 - Add a "browsing-context-did-set-embedder" observer notification, r=nika

Depends on D110336

Differential Revision: https://phabricator.services.mozilla.com/D111709

docshell/base/BrowsingContext.cpp

@@ -688,6 +688,13 @@ void BrowsingContext::SetEmbedderElement(Element* aEmbedder) {
   }
 
   mEmbedderElement = aEmbedder;
+
+  if (mEmbedderElement) {
+    if (nsCOMPtr<nsIObserverService> obs = services::GetObserverService()) {
+      obs->NotifyWhenScriptSafe(ToSupports(this),
+                                "browsing-context-did-set-embedder", nullptr);
+    }
+  }
 }
 
 void BrowsingContext::Embed() {