ash: Start/stop observing KeyboardController via ShellObserver

ash/common/keyboard/keyboard_observer_register.h

Index: ash/common/keyboard/keyboard_observer_register.h
diff --git a/ash/common/keyboard/keyboard_observer_register.h b/ash/common/keyboard/keyboard_observer_register.h
new file mode 100644
index 0000000000000000000000000000000000000000..11224a8cdb096d912dd79a8345b66b7f5c29b5f8
--- /dev/null
+++ b/ash/common/keyboard/keyboard_observer_register.h
@@ -0,0 +1,35 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ASH_COMMON_KEYBOARD_KEYBOAD_OBSERVER_REGISTER_H_
+#define ASH_COMMON_KEYBOARD_KEYBOAD_OBSERVER_REGISTER_H_
+
+#include "base/scoped_observer.h"
+
+namespace keyboard {
+class KeyboardController;
+class KeyboardControllerObserver;
+}
+
+namespace ash {
+
+class WmWindow;
+
+// Helper function to start/stop observing KeyboardController.
+// |keyboard_root_window| is the root window where KeyboardController changes
+// states.  |observer_root_window| is the root window relevant to the
+// |keyboard_observer|. If |keyboard_root_window| is different from
+// |observer_root_window|, this function is a no-op. Otherwise, the observing
+// starts if |keyboard_activated| is true and stops if |keyboard_activated| is
+// false.
+void UpdateKeyboardObserverFromStateChanged(
+    bool keyboard_activated,
+    WmWindow* keyboard_root_window,
+    WmWindow* observer_root_window,
+    ScopedObserver<keyboard::KeyboardController,
+                   keyboard::KeyboardControllerObserver>* keyboard_observer);
+
+}  // namespace ash
+
+#endif  // ASH_COMMON_KEYBOARD_KEYBOAD_OBSERVER_REGISTER_H_