Add Mojo constant for "ash" and replace hardcoded strings

mash/common/config.cc

Index: mash/common/config.cc
diff --git a/mash/common/config.cc b/mash/common/config.cc
index f4e14f7809364732bcc8324771866f4fc8370ae5..afb9e890e696ecdfc58b8481ffe310856fc0b54c 100644
--- a/mash/common/config.cc
+++ b/mash/common/config.cc
@@ -4,6 +4,11 @@
 
 #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 {
@@ -18,8 +23,11 @@ std::string GetWindowManagerServiceName() {
             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