chromeos: Makes AshTestBase/Helper target mash when appropriate

ash/mus/window_manager_application.h

Index: ash/mus/window_manager_application.h
diff --git a/ash/mus/window_manager_application.h b/ash/mus/window_manager_application.h
index f6596986b026d64519fabf4dfadfd218422e60a8..757e24bc4e71d7370343f23d415aace4e32de9b9 100644
--- a/ash/mus/window_manager_application.h
+++ b/ash/mus/window_manager_application.h
@@ -37,6 +37,9 @@ class AuraInit;
 }
 
 namespace ash {
+namespace test {
+class AshTestHelper;
+}
 namespace mus {
 
 class NetworkConnectDelegateMus;
@@ -51,6 +54,7 @@ class WindowManagerApplication : public service_manager::Service {
   WindowManager* window_manager() { return window_manager_.get(); }
 
  private:
+  friend class ash::test::AshTestHelper;
   friend class WmTestBase;
   friend class WmTestHelper;