bug 1575328 - Fix usage of nsIDocShellTreeItem in nsDocShell::SetMixedContentChannel. r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D58442

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

docshell/base/nsDocShell.cpp

@@ -4983,10 +4983,7 @@ nsDocShell::SetMixedContentChannel(nsIChannel* aMixedContentChannel) {
 #ifdef DEBUG
   // if the channel is non-null
   if (aMixedContentChannel) {
-    // Get the root docshell.
-    nsCOMPtr<nsIDocShellTreeItem> root;
-    GetInProcessSameTypeRootTreeItem(getter_AddRefs(root));
-    NS_WARNING_ASSERTION(root.get() == static_cast<nsIDocShellTreeItem*>(this),
+    NS_WARNING_ASSERTION(mBrowsingContext->IsTop(),
                          "Setting mMixedContentChannel on a docshell that is "
                          "not the root docshell");
   }