Moves TransientWindowClient to ui/aura/client and adds observer

ash/test/ash_test_helper.h

Index: ash/test/ash_test_helper.h
diff --git a/ash/test/ash_test_helper.h b/ash/test/ash_test_helper.h
index 5b14e0b03a7c37867186dd3beceb9f1bdca19ea8..9a0424c6544127ea077219de19b22eb5636442d1 100644
--- a/ash/test/ash_test_helper.h
+++ b/ash/test/ash_test_helper.h
@@ -24,6 +24,10 @@ namespace views {
 class ViewsDelegate;
 }
 
+namespace wm {
+class WMState;
+}
+
 namespace ash {
 namespace test {
 
@@ -80,6 +84,7 @@ class AshTestHelper {
   // Owned by ash::AcceleratorController
   TestScreenshotDelegate* test_screenshot_delegate_;
 
+  std::unique_ptr<::wm::WMState> wm_state_;
   std::unique_ptr<views::ViewsDelegate> views_delegate_;
 
 #if defined(OS_CHROMEOS)