Bug 1703692 - Store the latest embedder's permanent key on CanonicalBrowsingContext, r=nika,mccr8

And include it in Session Store flushes to avoid dropping updates in case the
browser is unavailable.

Differential Revision: https://phabricator.services.mozilla.com/D118385

docshell/base/BrowsingContext.cpp

@@ -693,6 +693,10 @@ void BrowsingContext::SetEmbedderElement(Element* aEmbedder) {
     MOZ_ALWAYS_SUCCEEDS(txn.Commit(this));
   }
 
+  if (XRE_IsParentProcess() && IsTopContent()) {
+    Canonical()->MaybeSetPermanentKey(aEmbedder);
+  }
+
   mEmbedderElement = aEmbedder;
 
   if (mEmbedderElement) {