Enable ash shell and unit tests in GN build.

ash/wm/overview/window_selector_unittest.cc

 // Copyright 2013 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 <algorithm>
 
 #include "ash/accessibility_delegate.h"
 #include "ash/drag_drop/drag_drop_controller.h"
 #include "ash/root_window_controller.h"
 #include "ash/screen_util.h"
@@ skipping 185 matching lines @@
         window_selector_->grid_list_[index]->window_list_.get();
     }
 
   const aura::Window* GetSelectedWindow() {
     WindowSelector* ws = ash::Shell::GetInstance()->
         window_selector_controller()->window_selector_.get();
     return ws->grid_list_[ws->selected_grid_index_]->
         SelectedWindow()->SelectionWindow();
   }
 
-  const bool selection_widget_active() {
+  bool selection_widget_active() {
     WindowSelector* ws = ash::Shell::GetInstance()->
         window_selector_controller()->window_selector_.get();
     return ws->grid_list_[ws->selected_grid_index_]->is_selecting();
   }
 
   bool showing_filter_widget() {
     WindowSelector* ws = ash::Shell::GetInstance()->
         window_selector_controller()->window_selector_.get();
     return ws->text_filter_widget_->GetNativeWindow()->layer()->
         GetTargetTransform().IsIdentity();
@@ skipping 972 matching lines @@
   // Switch to overview mode.
   ToggleOverview();
   ASSERT_TRUE(IsSelecting());
 
   // Tap should now exit overview mode.
   generator.GestureTapAt(point_in_background_page);
   EXPECT_FALSE(IsSelecting());
 }
 
 }  // namespace ash