Makes InputMethodMus deal with no connection

ui/aura/mus/window_tree_host_mus.h

Index: ui/aura/mus/window_tree_host_mus.h
diff --git a/ui/aura/mus/window_tree_host_mus.h b/ui/aura/mus/window_tree_host_mus.h
index ea444fcb03f4a60edfd997d2c8273cbfeda2fc6e..40473cf39dc9ed839fa74ab6e91884b0a7b5f4ff 100644
--- a/ui/aura/mus/window_tree_host_mus.h
+++ b/ui/aura/mus/window_tree_host_mus.h
@@ -32,10 +32,10 @@ class AURA_EXPORT WindowTreeHostMus : public aura::WindowTreeHostPlatform {
  public:
   WindowTreeHostMus(
       std::unique_ptr<WindowPortMus> window_port,
-      WindowTreeHostMusDelegate* delegate,
+      WindowTreeClient* window_tree_client,
       int64_t display_id,
       const std::map<std::string, std::vector<uint8_t>>* properties = nullptr);
-  WindowTreeHostMus(
+  explicit WindowTreeHostMus(
       WindowTreeClient* window_tree_client,
       const std::map<std::string, std::vector<uint8_t>>* properties = nullptr);