ash: Use SessionController instead of SessionStateDelegate

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 1128bcdcb77c600333dfcfbc4861ed7f7d8d1c2c..82d85da8e62804a03ce4dd95b3e167df49462979 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -1592,8 +1592,10 @@ void WindowTreeClient::SetFrameDecorationValues(
 
 void WindowTreeClient::SetNonClientCursor(Window* window,
                                           ui::mojom::Cursor cursor_id) {
-  window_manager_internal_client_->WmSetNonClientCursor(
-      WindowMus::Get(window)->server_id(), cursor_id);
+  if (window_manager_internal_client_) {

[xiyuan, 2017/03/17 07:08:16]

Needed for WorkspaceLayoutManagerSoloTest.NotResizeWhenScreenIsLocked running as
part of mash_unittests. |window_manager_internal_client_| is not bound somehow.

[James Cook, 2017/03/17 17:14:37]

This seems fine -- we've checking it for null everywhere else. :-)

+    window_manager_internal_client_->WmSetNonClientCursor(
+        WindowMus::Get(window)->server_id(), cursor_id);
+  }
 }
 
 void WindowTreeClient::AddAccelerators(