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..f0a3721f36d78281a0910a1ab2e433fdcedbac01 100644
--- a/mash/common/config.cc
+++ b/mash/common/config.cc
@@ -6,6 +6,10 @@
 
 #include "base/command_line.h"
 
+#if defined(USE_ASH)

[sky, 2017/02/24 21:36:34]

Sorry, we want to get rid of USE_ASH. It'll be replaced by OS_CHROMEOS. So,
could you use OS_CHROMEOS here and below?

[fwang, 2017/02/24 22:00:08]

Done.

+#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(USE_ASH)
+  return ash::mojom::kServiceName;
+#else

[tonikitoo, 2017/02/24 19:03:21]

Actually !USE_ASH (or !OS_CHROMEOS) to not work with simple_wm any longer
(Because of ScreenManagerOzoneExternal not calling back to
DisplayManager::OnAcceleratedWidgetAvailable).

So, in other words, one can not produce a LINUX_OS build and run it with
window-manager=simple_wm in ToT.

I believe we can just omit the #if/#else here.

[fwang, 2017/02/24 19:19:58]

On 2017/02/24 19:03:21, tonikitoo wrote:

Is this GetWindowManagerServiceName() needed for platforms where USE_ASH=0? If
not, I guess we should disable the compilation of that config.cc file when
USE_ASH=0, remove the #ifdef here and only call GetWindowManagerServiceName() in
other places when USE_ASH=1. WDYT?

[sky, 2017/02/24 21:18:43]

It is used on platforms where USE_ASH=0. Say windows.

+  LOG(FATAL)
+      << "You must specify a window manager e.g. --window-manager=simple_wm";
+  return std::string();
+#endif
 }
 
 }  // namespace common