chromeos: Makes AshTestBase/Helper target mash when appropriate

ash/mus/bridge/wm_shell_mus.h

Index: ash/mus/bridge/wm_shell_mus.h
diff --git a/ash/mus/bridge/wm_shell_mus.h b/ash/mus/bridge/wm_shell_mus.h
index 6894a323316be4715f491079fa2cb8ecb42a80d7..e94d2881fe0840efe39e6198bde17c3a5ccb1bd9 100644
--- a/ash/mus/bridge/wm_shell_mus.h
+++ b/ash/mus/bridge/wm_shell_mus.h
@@ -36,10 +36,13 @@ class WmShellMusTestApi;
 // WmShell implementation for mus.
 class WmShellMus : public WmShell {
  public:
+  // If |create_session_state_delegate_stub| is true SessionStateDelegateStub is
+  // created. If false, the SessionStateDelegate from Shell is used.
   WmShellMus(WmWindow* primary_root_window,
              std::unique_ptr<ShellDelegate> shell_delegate,
              WindowManager* window_manager,
-             views::PointerWatcherEventRouter* pointer_watcher_event_router);
+             views::PointerWatcherEventRouter* pointer_watcher_event_router,
+             bool create_session_state_delegate_stub);

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

Can we just rely on the shell_delegate for this?
ShellDelegateMus::CreateSessionStateDelegate already makes a stub.

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

Shell::Init() creates a SessionStateDelegate:

  session_state_delegate_.reset(
      wm_shell_->delegate()->CreateSessionStateDelegate());

In test code this ends up in TestShellDelegate::CreateSessionStateDelegate(),
which creates a TestSessionStateDelegate. Test code depends upon
TestSessionStateDelegate:

// static
TestSessionStateDelegate* AshTestHelper::GetTestSessionStateDelegate() {
  CHECK(WmShell::HasInstance());
  return static_cast<TestSessionStateDelegate*>(
      WmShell::Get()->GetSessionStateDelegate());
}

In other words the test code wants to use TestSessionStateDelegate, which I made
it get by having WmShellMus not create a SessionStateDelegate so that in:

SessionStateDelegate* WmShellMus::GetSessionStateDelegate() {
  return session_state_delegate_
             ? session_state_delegate_.get()
             : Shell::GetInstance()->session_state_delegate();
}

WmShellMus::session_state_delegate_ is null, and the code falls back to that of
Shell, which is TestSessionStateDelegate.

It seems the right fix is that both Shell and WmShellMus shouldn't create
SessionStateDelegate, only Shell. That's a bigger change though.

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

Got it, it'd be nice to summarize that in a bug or comment. It's slightly
confusing without that explainer.

   ~WmShellMus() override;
 
   static WmShellMus* Get();