bug 1207862 - make it possible to traverse down the tree of proxied documents r=davidb

accessible/ipc/DocAccessibleParent.h

@@ -123,6 +123,10 @@ public:
   const ProxyAccessible* GetAccessible(uintptr_t aID) const
     { return const_cast<DocAccessibleParent*>(this)->GetAccessible(aID); }
 
+  size_t ChildDocCount() const { return mChildDocs.Length(); }
+  const DocAccessibleParent* ChildDocAt(size_t aIdx) const
+    { return mChildDocs[aIdx]; }
+
 private:
 
   class ProxyEntry : public PLDHashEntryHdr