Add Mojo constant for "ash" and replace hardcoded strings

ash/mus/app_launch_unittest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "ash/public/interfaces/constants.mojom.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/run_loop.h"
 #include "mash/quick_launch/public/interfaces/constants.mojom.h"
 #include "services/service_manager/public/cpp/service_test.h"
 #include "services/ui/public/interfaces/constants.mojom.h"
 #include "services/ui/public/interfaces/window_server_test.mojom.h"
 
 namespace ash {
 namespace mus {
@@ skipping 11 matching lines @@
  private:
   void SetUp() override {
     base::CommandLine::ForCurrentProcess()->AppendSwitch("use-test-config");
     ServiceTest::SetUp();
   }
 
   DISALLOW_COPY_AND_ASSIGN(AppLaunchTest);
 };
 
 TEST_F(AppLaunchTest, TestQuickLaunch) {
-  connector()->Connect("ash");
+  connector()->Connect(mojom::kServiceName);
   connector()->Connect(mash::quick_launch::mojom::kServiceName);
 
   ui::mojom::WindowServerTestPtr test_interface;
   connector()->BindInterface(ui::mojom::kServiceName, &test_interface);
 
   base::RunLoop run_loop;
   bool success = false;
   test_interface->EnsureClientHasDrawnWindow(
       mash::quick_launch::mojom::kServiceName,
       base::Bind(&RunCallback, &success, run_loop.QuitClosure()));
   run_loop.Run();
   EXPECT_TRUE(success);
 }
 
 }  // namespace mus
 }  // namespace ash