Add WidgetTest::GetAllWidgets() to find dialogs created by TestBrowserDialog.

ui/views/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 import("//ui/base/ui_features.gni")
 import("//ui/ozone/ozone.gni")
 
@@ skipping 793 matching lines @@
       "corewm/tooltip_controller_test_helper.h",
       "test/desktop_test_views_delegate_aura.cc",
       "test/test_views_delegate_aura.cc",
       "test/views_test_helper_aura.cc",
       "test/views_test_helper_aura.h",
       "test/widget_test_aura.cc",
     ]
     deps += [
       "//ui/aura",
       "//ui/aura:test_support",
+      "//ui/views/mus",
       "//ui/wm",
     ]
     if (use_x11 && !is_chromeos) {
       sources += [
         "test/desktop_screen_x11_test_api.cc",
         "test/desktop_screen_x11_test_api.h",
         "test/test_desktop_screen_x11.cc",
         "test/test_desktop_screen_x11.h",
         "test/ui_controls_factory_desktop_aurax11.cc",
         "test/ui_controls_factory_desktop_aurax11.h",
@@ skipping 79 matching lines @@
     "controls/table/test_table_model.h",
     "controls/textfield/textfield_model_unittest.cc",
     "controls/tree/tree_view_unittest.cc",
     "event_monitor_unittest.cc",
     "focus/focus_manager_unittest.cc",
     "focus/focus_traversal_unittest.cc",
     "layout/box_layout_unittest.cc",
     "layout/fill_layout_unittest.cc",
     "layout/grid_layout_unittest.cc",
     "rect_based_targeting_utils_unittest.cc",
+    "test/widget_test_unittest.cc",
     "view_model_unittest.cc",
     "view_model_utils_unittest.cc",
     "view_targeter_unittest.cc",
     "view_unittest.cc",
     "widget/native_widget_mac_accessibility_unittest.mm",
     "widget/native_widget_mac_unittest.mm",
     "widget/native_widget_unittest.cc",
     "widget/root_view_unittest.cc",
     "widget/widget_unittest.cc",
     "widget/window_reorderer_unittest.cc",
@@ skipping 149 matching lines @@
       "//ui/base:test_support",
       "//ui/compositor",
       "//ui/events:test_support",
       "//ui/gl:test_support",
       "//ui/resources",
       "//ui/resources:ui_test_pak",
       "//ui/strings",
     ]
   }
 }