Fixes a DCHECK in tools_menu_button_observer_bridge during tests

components/reading_list/core/reading_list_model_unittest.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 "components/reading_list/core/reading_list_model.h"
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "base/test/simple_test_clock.h"
 #include "components/reading_list/core/reading_list_model_impl.h"
@@ skipping 296 matching lines @@
 };
 
 // Tests creating an empty model.
 TEST_F(ReadingListModelTest, EmptyLoaded) {
   EXPECT_TRUE(model_->loaded());
   AssertObserverCount(1, 0, 0, 0, 0, 0, 0, 0, 0);
   EXPECT_EQ(0ul, UnreadSize());
   EXPECT_EQ(0ul, ReadSize());
   model_->Shutdown();
   EXPECT_FALSE(model_->loaded());
+  // Shutdown() does not delete the model observer. Verify that deleting the
+  // model will delete the model observer.
+  model_.reset();
   AssertObserverCount(1, 0, 0, 1, 0, 0, 0, 0, 0);
 }
 
 // Tests load model.
 TEST_F(ReadingListModelTest, ModelLoaded) {
   ClearCounts();
   auto clock = base::MakeUnique<base::SimpleTestClock>();
   auto storage = base::MakeUnique<TestReadingListStorage>(this, clock.get());
   storage->AddSampleEntries();
   SetStorage(std::move(storage), std::move(clock));
@@ skipping 411 matching lines @@
   model_->SetReadStatus(gurl, true);
   const ReadingListEntry* entry = model_->GetEntryByURL(gurl);
   EXPECT_EQ(entry->Title(), "This title contains new line characters");
   model_->SetEntryTitle(gurl, "test");
   EXPECT_EQ(entry->Title(), "test");
   model_->SetEntryTitle(gurl, title);
   EXPECT_EQ(entry->Title(), "This title contains new line characters");
 }
 
 }  // namespace