Bug 1621779 - Make sure BrowsingContext exists before using it. r=alwu

A declare of a pointer in the init-statement within if need to be
checked if it's a NULL before it's being used, to avoid a potential
crash.

To prevent any goes wrong in mMediaSessionInfoMap and lead to create a
null BrowsingContext pointer from it, using an assertion is more
straightforward.

Differential Revision: https://phabricator.services.mozilla.com/D66491

--HG--
extra : moz-landing-system : lando

dom/media/mediasession/MediaSessionController.cpp

@@ -105,8 +105,9 @@ void MediaSessionController::UpdateActiveMediaSessionContextId() {
 
   for (auto iter = mMediaSessionInfoMap.ConstIter(); !iter.Done();
        iter.Next()) {
-    if (RefPtr<BrowsingContext> bc = BrowsingContext::Get(iter.Key());
-        bc->IsTopContent()) {
+    RefPtr<BrowsingContext> bc = BrowsingContext::Get(iter.Key());
+    MOZ_ASSERT(bc);
+    if (bc->IsTopContent()) {
       candidateId = iter.Key();
       break;
     }