Bug 1405065: Implement accNavigate in Windows a11y LazyInstantiator. r=MarcoZ

The first accessibility client to ask for the root accessible will get a surrogate LazyInstantiator object. That instantiates accessibility if appropriate and then passes calls through.
However, it previously didn't implement accNavigate, which broke things for clients that used this on the root.
In particular, this was bad for NAVRELATION_EMBEDS, which is used by some clients to retrieve the tab document.
This patch simply passes accNavigate through to the real accessible, just as is already done for other methods.

MozReview-Commit-ID: KIoA9BwMDhc

--HG--
extra : rebase_source : 7514a0a6fb3750df128ad1e790e1fd1978c031e8

accessible/windows/msaa/LazyInstantiator.cpp

@@ -812,7 +812,12 @@ HRESULT
 LazyInstantiator::accNavigate(long navDir, VARIANT varStart,
                               VARIANT *pvarEndUpAt)
 {
-  return E_NOTIMPL;
+  if (!pvarEndUpAt) {
+    return E_INVALIDARG;
+  }
+
+  RESOLVE_ROOT;
+  return mWeakAccessible->accNavigate(navDir, varStart, pvarEndUpAt);
 }
 
 HRESULT