Enable chrome.automation.getDesktop on all aura platforms.

ui/views/accessibility/ax_aura_obj_cache.cc

Index: ui/views/accessibility/ax_aura_obj_cache.cc
diff --git a/ui/views/accessibility/ax_aura_obj_cache.cc b/ui/views/accessibility/ax_aura_obj_cache.cc
index 05f18686fc4154e7b4c22f090c63b620325662c8..8d5937b39c1b702d1a927af9cf1f82d05c738015 100644
--- a/ui/views/accessibility/ax_aura_obj_cache.cc
+++ b/ui/views/accessibility/ax_aura_obj_cache.cc
@@ -76,6 +76,15 @@ void AXAuraObjCache::Remove(int32 id) {
   delete obj;
 }
 
+void AXAuraObjCache::GetTopLevelWindows(
+    std::vector<AXAuraObjWrapper*>* children) {
+  for (std::map<aura::Window*, int32>::iterator it = window_to_id_map_.begin();
+       it != window_to_id_map_.end(); ++it) {
+    if (!it->first->parent())
+      children->push_back(GetOrCreate(it->first));
+  }
+}
+
 AXAuraObjCache::AXAuraObjCache() : current_id_(1), is_destroying_(false) {
 }