Bug 1858083. Avoid needless addref in Document::GetBrowsingContext. r=smaug

It's a weak pointer but presumably can only die based on main thread actions. The GetBrowsingContext getter is just a plain getter, returning a pointer.

Although small this does show up in the profile in bug 1857094. It's called from some common layout/painting functions.

Differential Revision: https://phabricator.services.mozilla.com/D190559

dom/base/Document.cpp

@@ -16696,8 +16696,8 @@ void Document::SetUserHasInteracted() {
 }
 
 BrowsingContext* Document::GetBrowsingContext() const {
-  nsCOMPtr<nsIDocShell> docshell(mDocumentContainer);
-  return docshell ? docshell->GetBrowsingContext() : nullptr;
+  return mDocumentContainer ? mDocumentContainer->GetBrowsingContext()
+                            : nullptr;
 }
 
 void Document::NotifyUserGestureActivation() {