Bug 1761865 - Change assert to warning; follow up to bug 1753298. r=padenot

Differential Revision: https://phabricator.services.mozilla.com/D142324

dom/media/BaseMediaResource.cpp

@@ -136,8 +136,7 @@ void BaseMediaResource::SetLoadInBackground(bool aLoadInBackground) {
     } else {
       loadFlags &= ~nsIRequest::LOAD_BACKGROUND;
     }
-    rv = ModifyLoadFlags(loadFlags);
-    NS_ASSERTION(NS_SUCCEEDED(rv), "ModifyLoadFlags() failed!");
+    Unused << NS_WARN_IF(NS_FAILED(ModifyLoadFlags(loadFlags)));
   }
 }