bug 1196460 - add class for wrapping proxies of document accessibles r=surkov

For now this isn't really different from the class used to wrap
HyperTextAccessibles.  However we will need to store extra data to map IDs to
accessibles when we implement events.

accessible/windows/ProxyWrappers.h

@@ -42,6 +42,13 @@ public:
   virtual void Shutdown() override { mBits.proxy = nullptr; }
 };
 
+class DocProxyAccessibleWrap : public HyperTextProxyAccessibleWrap
+{
+public:
+  DocProxyAccessibleWrap(ProxyAccessible* aProxy) :
+    HyperTextProxyAccessibleWrap(aProxy) {}
+};
+
 template<typename T>
 inline ProxyAccessible*
 HyperTextProxyFor(T* aWrapper)