Bug 1408638: Ensure accessible isn't defunct in Windows RootAccessibleWrap::accNavigate. r=MarcoZ

Bug 1407475 added support for accNavigate(NAVRELATION_EMBEDS) for e10s.
However, it's possible for a client to call this on the root accessible for a window which was since closed.
Therefore, we must check whether the root accessible is defunct before trying to retrieve the tab document.

MozReview-Commit-ID: 9iR6Kvzu5Mb

--HG--
extra : rebase_source : 82afdecf915cd09cc3eaed948587b87d8ac4173b

accessible/windows/msaa/RootAccessibleWrap.cpp

@@ -124,6 +124,9 @@ RootAccessibleWrap::accNavigate(
   if (!pvarEndUpAt) {
     return E_INVALIDARG;
   }
+  if (IsDefunct()) {
+    return CO_E_OBJNOTCONNECTED;
+  }
 
   Accessible* target = nullptr;
   // Get the document in the active tab.