Eliminates all ash dependencies from WindowModalityController.

ui/aura/test/test_activation_client.cc

Index: ui/aura/test/test_activation_client.cc
===================================================================
--- ui/aura/test/test_activation_client.cc	(revision 170295)
+++ ui/aura/test/test_activation_client.cc	(working copy)
@@ -69,6 +69,10 @@
   return active_windows_.back();
 }
 
+Window* TestActivationClient::GetActivatableWindow(Window* window) {
+  return NULL;
+}
+
 bool TestActivationClient::OnWillFocusWindow(Window* window,
                                              const ui::Event* event) {
   return true;