Makes ScreenDimmer deal with the shell going away

ash/wm/screen_dimmer_unittest.cc

 // Copyright (c) 2012 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 "ash/common/wm/screen_dimmer.h"
 
 #include <memory>
 
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/wm/window_dimmer.h"
@@ skipping 102 matching lines @@
   dimmer_->set_at_bottom(true);
   dimmer_->SetDimming(true);
 
   dim_iter = std::find(root_window->children().begin(),
                        root_window->children().end(), GetDimWindow());
   ASSERT_TRUE(dim_iter != root_window->children().end());
   // Dom layer is at the bottom.
   EXPECT_EQ(*dim_iter, *root_window->children().begin());
 }
 
+// See description above TEST_F for details.
+class ScreenDimmerShellDestructionTest : public AshTestBase {
+ public:
+  ScreenDimmerShellDestructionTest() {}
+  ~ScreenDimmerShellDestructionTest() override {}
+
+  void TearDown() override {
+    ScreenDimmer screen_dimmer(ScreenDimmer::Container::ROOT);
+    AshTestBase::TearDown();
+    // ScreenDimmer is destroyed *after* the shell.
+  }
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ScreenDimmerShellDestructionTest);
+};
+
+// This test verifies ScreenDimmer can be destroyed after the shell. The
+// interesting part of this test is in TearDown(), which creates a ScreenDimmer
+// that is deleted after WmShell.
+TEST_F(ScreenDimmerShellDestructionTest, DontCrashIfScreenDimmerOutlivesShell) {
+}
+
 }  // namespace test
 }  // namespace ash