chromeos: Makes AshTestBase/Helper target mash when appropriate

ash/mus/window_manager.h

Index: ash/mus/window_manager.h
diff --git a/ash/mus/window_manager.h b/ash/mus/window_manager.h
index 945bb5bdca6ec2ac04884d435bb28f152777f35a..8565d6450c126f8cad62151a6b16706664e4c17b 100644
--- a/ash/mus/window_manager.h
+++ b/ash/mus/window_manager.h
@@ -45,6 +45,10 @@ class RootWindowController;
 class ScreenMus;
 class ShellDelegate;
 
+namespace test {
+class AshTestHelper;
+}
+
 namespace mus {
 
 class AcceleratorHandler;
@@ -102,6 +106,7 @@ class WindowManager : public aura::WindowManagerDelegate,
       std::unique_ptr<aura::WindowTreeHostMus> window_tree_host);
 
  private:
+  friend class ash::test::AshTestHelper;
   friend class WmTestHelper;
 
   using RootWindowControllers = std::set<std::unique_ptr<RootWindowController>>;
@@ -194,6 +199,9 @@ class WindowManager : public aura::WindowManagerDelegate,
   // Only set in tests. If non-null this is used as the shell delegate.
   std::unique_ptr<ShellDelegate> shell_delegate_for_test_;
 
+  // See WmShellMus's constructor for details. Tests may set to false.
+  bool create_session_state_delegate_stub_for_test_ = true;
+
   DISALLOW_COPY_AND_ASSIGN(WindowManager);
 };