Add --keyboard-usability-test flag and always show keyboard unless manually hide

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index b87a9467f5272313bea319e337026640b6d364ae..3f404cf7598c5f8904c22a154da08b1e67496366 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -337,6 +337,8 @@ void Shell::OnLoginStateChanged(user::LoginStatus status) {
     // TODO(bshe): Primary root window controller may not be the controller to
     // attach virtual keyboard. See http://crbug.com/303429
     InitKeyboard(GetPrimaryRootWindowController());
+    GetPrimaryRootWindowController()->ActivateKeyboard(
+        keyboard_controller_.get());
   }
   FOR_EACH_OBSERVER(ShellObserver, observers_, OnLoginStateChanged(status));
 }
@@ -910,7 +912,6 @@ void Shell::InitKeyboard(internal::RootWindowController* root) {
         delegate_->CreateKeyboardControllerProxy();
     keyboard_controller_.reset(
         new keyboard::KeyboardController(proxy));
-    root->ActivateKeyboard(keyboard_controller_.get());
   }
 }