ARC: Avoid duplicate observers in IME service

components/arc/ime/arc_ime_service.h

Index: components/arc/ime/arc_ime_service.h
diff --git a/components/arc/ime/arc_ime_service.h b/components/arc/ime/arc_ime_service.h
index b29548c1dfa7109dd90a57c494a828af9564a77a..52387b91bc1fbd4517f11f72f5836871b9f127f5 100644
--- a/components/arc/ime/arc_ime_service.h
+++ b/components/arc/ime/arc_ime_service.h
@@ -22,6 +22,10 @@
 #include "ui/keyboard/keyboard_controller_observer.h"
 
 namespace aura {
+namespace client {
+class FocusClient;
+}
+
 class Window;
 }
 
@@ -114,10 +118,10 @@ class ArcImeService : public ArcService,
   gfx::Rect cursor_rect_;
   bool has_composition_text_;
 
-  aura::WindowTracker observing_root_windows_;
   aura::WindowTracker arc_windows_;
   aura::WindowTracker focused_arc_window_;
 
+  aura::client::FocusClient* focus_client_;
   keyboard::KeyboardController* keyboard_controller_;
 
   ui::InputMethod* test_input_method_;