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..b13e15f8e8594c81ed85c1c680f51af1a5610b74 100644
--- a/mash/common/config.cc
+++ b/mash/common/config.cc
@@ -6,6 +6,10 @@
 
 #include "base/command_line.h"
 
+#if defined(OS_CHROMEOS)
+#include "ash/public/interfaces/constants.mojom.h"  // nogncheck
+#endif
+
 namespace mash {
 namespace common {
 
@@ -18,8 +22,13 @@ std::string GetWindowManagerServiceName() {
             kWindowManagerSwitch);
     return service_name;
   }
-  // TODO(beng): move this constant to a mojom file in //ash.
-  return "ash";
+#if defined(OS_CHROMEOS)
+  return ash::mojom::kServiceName;
+#else
+  LOG(FATAL)
+      << "You must specify a window manager e.g. --window-manager=simple_wm";
+  return std::string();
+#endif
 }
 
 }  // namespace common