Moves TransientWindowClient to ui/aura/client and adds observer

ash/test/ash_test_helper.cc

Index: ash/test/ash_test_helper.cc
diff --git a/ash/test/ash_test_helper.cc b/ash/test/ash_test_helper.cc
index d3377a6d0e0c7dc0fdca634d6eaa679082acecc4..f882d7c29da5a12c5005da0f521509e7d9bad362 100644
--- a/ash/test/ash_test_helper.cc
+++ b/ash/test/ash_test_helper.cc
@@ -20,6 +20,7 @@
 #include "ash/test/shell_test_api.h"
 #include "ash/test/test_screenshot_delegate.h"
 #include "ash/test/test_shell_delegate.h"
+#include "base/memory/ptr_util.h"
 #include "base/run_loop.h"
 #include "ui/aura/env.h"
 #include "ui/aura/input_state_lookup.h"
@@ -34,6 +35,7 @@
 #include "ui/message_center/message_center.h"
 #include "ui/wm/core/capture_controller.h"
 #include "ui/wm/core/cursor_manager.h"
+#include "ui/wm/core/wm_state.h"
 
 #if defined(OS_CHROMEOS)
 #include "ash/system/chromeos/screen_layout_observer.h"
@@ -73,6 +75,7 @@ AshTestHelper::~AshTestHelper() {}
 void AshTestHelper::SetUp(bool start_session,
                           MaterialDesignController::Mode material_mode) {
   display::ResetDisplayIdForTest();
+  wm_state_ = base::MakeUnique<::wm::WMState>();
   views_delegate_ = ash_test_environment_->CreateViewsDelegate();
 
   // Disable animations during tests.