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.

[msw, 2017/02/19 21:49:20]

This is a bit confusing... tests may set this "create*for_test_" flag to false,
but otherwise it's true by default? I hope we can instead rely on using the
correct shell delegate (directly above is a |shell_delegate_for_test_| member),
which does the right thing in CreateSessionStateDelegate.

[sky, 2017/02/21 17:06:50]

Prior to my change the code always created a SessionStateDelegateStub. I needed
to turn that off in tests, hence naming it
create_session_state_delegate_stub_for_test_. I could invert the meaning, e.g.
dont_create_session_state_delegate_for_test_ if you prefer.

[msw, 2017/02/22 06:25:48]

No, it's fine as-is... hopefully the stub will go away soon.

+  bool create_session_state_delegate_stub_for_test_ = true;
+
   DISALLOW_COPY_AND_ASSIGN(WindowManager);
 };