ash: Start/stop observing KeyboardController via ShellObserver

ash/common/shell_observer.h

Index: ash/common/shell_observer.h
diff --git a/ash/common/shell_observer.h b/ash/common/shell_observer.h
index facc29b9e2f289e6f2b7467ea49ebe413d625a70..e9cd6174354e57442c51c9b403c2fa32d01be262 100644
--- a/ash/common/shell_observer.h
+++ b/ash/common/shell_observer.h
@@ -67,8 +67,9 @@ class ASH_EXPORT ShellObserver {
   // animating but have been restored.
   virtual void OnMaximizeModeEnded() {}
 
-  // Called when keyboard is activated/deactivated.
-  virtual void OnVirtualKeyboardStateChanged(bool activated) {}
+  // Called when keyboard is activated/deactivated in |root_window|.
+  virtual void OnVirtualKeyboardStateChanged(bool activated,
+                                             WmWindow* root_window) {}
 
   // Called at the end of Shell::Init.
   virtual void OnShellInitialized() {}