Converts mash to use Shell

ash/mus/keyboard_ui_mus.cc

Index: ash/mus/keyboard_ui_mus.cc
diff --git a/ash/mus/keyboard_ui_mus.cc b/ash/mus/keyboard_ui_mus.cc
index 2f745aa61f453a71be29ec38bd0d55ca6d4c6ef0..83c85cb7b6af130d0644f92836762b5496f31d8c 100644
--- a/ash/mus/keyboard_ui_mus.cc
+++ b/ash/mus/keyboard_ui_mus.cc
@@ -29,17 +29,20 @@ std::unique_ptr<KeyboardUI> KeyboardUIMus::Create(
 }
 
 void KeyboardUIMus::Hide() {
-  keyboard_->Hide();
+  if (keyboard_)
+    keyboard_->Hide();
 }
 
 void KeyboardUIMus::Show() {
-  keyboard_->Show();
+  if (keyboard_)
+    keyboard_->Show();
 }
 
 void KeyboardUIMus::ShowInDisplay(const int64_t display_id) {
   // TODO(yhanada): Send display id after adding a display_id argument to
   // |Keyboard::Show()| in keyboard.mojom. See crbug.com/585253.
-  keyboard_->Show();
+  if (keyboard_)
+    keyboard_->Show();
 }
 
 bool KeyboardUIMus::IsEnabled() {