Add Mojo constant for "ash" and replace hardcoded strings

mash/common/config.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 "mash/common/config.h"
 
+#if defined(USE_ASH)
+#include "ash/public/interfaces/constants.mojom.h"  // nogncheck
+#else
+#include "mash/simple_wm/public/interfaces/constants.mojom.h"  // nogncheck
+#endif
 #include "base/command_line.h"
 
 namespace mash {
 namespace common {
 
 const char kWindowManagerSwitch[] = "window-manager";
 
 std::string GetWindowManagerServiceName() {
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(kWindowManagerSwitch)) {
     std::string service_name =
         base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
             kWindowManagerSwitch);
     return service_name;
   }
-  // TODO(beng): move this constant to a mojom file in //ash.
-  return "ash";
+#if defined(USE_ASH)
+  return ash::mojom::kServiceName;
+#else
+  return mash::simple_wm::mojom::kServiceName;

[tonikitoo, 2017/02/21 14:29:46]

I believe there is a slightly behavior change here. Before the patch, for
non-ChromeOS (and non-ash) builds, it would just fail ('ash' would not start).

With the patch, even if the command line does not include
"--window-manager=simple_vm" it would launch it?

[fwang, 2017/02/21 14:34:20]

Yes, it only makes sense to return "ash" when ash is built. If it is not, we
default to simple_wm which is always built with mash. Alternatively, if we want
to preserve the failure behavior, we could just return an empty string or
terminate with a FATAL error...

[fwang, 2017/02/22 07:12:59]

Done in patch set 3.

+#endif
 }
 
 }  // namespace common
 }  // namespace mash