Bug 1783098: Fix COM reference leak in MsaaRootAccessible::get_accFocus. r=morgan

NativeAccessible AddRefs, so we don't want to implicitly AddRef again when assigning to a RefPtr.

Differential Revision: https://phabricator.services.mozilla.com/D153699

accessible/windows/msaa/MsaaRootAccessible.cpp

@@ -139,7 +139,7 @@ MsaaRootAccessible::get_accFocus(
   if (!docProxy) {
     return hr;
   }
-  RefPtr<IDispatch> docDisp = NativeAccessible(docProxy);
+  RefPtr<IDispatch> docDisp = already_AddRefed(NativeAccessible(docProxy));
   if (!docDisp) {
     return E_FAIL;
   }