Enable ash window cycle UI by default.

ui/app_list/presenter/app_list_presenter_impl.cc

 // Copyright 2016 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.
 
 #include "ui/app_list/presenter/app_list_presenter_impl.h"
 
 #include "ui/app_list/app_list_constants.h"
 #include "ui/app_list/app_list_switches.h"
 #include "ui/app_list/pagination_model.h"
 #include "ui/app_list/presenter/app_list_presenter_delegate_factory.h"
@@ skipping 62 matching lines @@
 
 void AppListPresenterImpl::Dismiss() {
   if (!is_visible_)
     return;
 
   // If the app list is currently visible, there should be an existing view.
   DCHECK(view_);
 
   is_visible_ = false;
 
-  // Our widget is currently active. When the animation completes we'll hide
-  // the widget, changing activation. If a menu is shown before the animation
-  // completes then the activation change triggers the menu to close. By
-  // deactivating now we ensure there is no activation change when the
-  // animation completes and any menus stay open.
-  view_->GetWidget()->Deactivate();
+  // The dismissal may have occurred in response to the app list losing
+  // activation. Otherwise, our widget is currently active. When the animation
+  // completes we'll hide the widget, changing activation. If a menu is shown
+  // before the animation completes then the activation change triggers the menu
+  // to close. By deactivating now we ensure there is no activation change when
+  // the animation completes and any menus stay open.
+  if (view_->GetWidget()->IsActive())

[Evan Stade, 2016/08/12 22:51:58]

This is necessary because otherwise Alt+Tabbing while the app list is up causes
the app list to deactivate and pass activation to some other random window,
stealing activation from the window cycle ui.

I've added test coverage for this in
WindowCycleControllerTest.SelectingHidesAppList

+    view_->GetWidget()->Deactivate();
 
   presenter_delegate_->OnDismissed();
   ScheduleAnimation();
 }
 
 void AppListPresenterImpl::ToggleAppList(int64_t display_id) {
   if (IsVisible()) {
     Dismiss();
     return;
   }
@@ skipping 163 matching lines @@
   } else if (should_snap_back_) {
     should_snap_back_ = false;
     ui::ScopedLayerAnimationSettings animation(widget_animator);
     animation.SetTransitionDuration(
         base::TimeDelta::FromMilliseconds(kOverscrollPageTransitionDurationMs));
     widget->SetBounds(view_bounds_);
   }
 }
 
 }  // namespace app_list