Fix WebStateListObserver::WebStateDetachedAt() parameter lifetime.

ios/shared/chrome/browser/tabs/web_state_list_unittest.mm

 // Copyright 2017 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 "ios/shared/chrome/browser/tabs/web_state_list.h"
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/supports_user_data.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list_observer.h"
@@ skipping 310 matching lines @@
   web_state_list_.InsertWebState(1, CreateWebState(kURL1), nullptr);
   web_state_list_.InsertWebState(2, CreateWebState(kURL2), nullptr);
 
   // Sanity check before closing WebState.
   EXPECT_EQ(3, web_state_list_.count());
   EXPECT_EQ(kURL0, web_state_list_.GetWebStateAt(0)->GetVisibleURL().spec());
   EXPECT_EQ(kURL1, web_state_list_.GetWebStateAt(1)->GetVisibleURL().spec());
   EXPECT_EQ(kURL2, web_state_list_.GetWebStateAt(2)->GetVisibleURL().spec());
 
   observer_.ResetStatistics();
-  web_state_list_.DetachWebStateAt(0);
+  std::unique_ptr<web::WebState> old_web_state(
+      web_state_list_.DetachWebStateAt(0));
 
   EXPECT_TRUE(observer_.web_state_detached_called());
   EXPECT_EQ(2, web_state_list_.count());
   EXPECT_EQ(kURL1, web_state_list_.GetWebStateAt(0)->GetVisibleURL().spec());
   EXPECT_EQ(kURL2, web_state_list_.GetWebStateAt(1)->GetVisibleURL().spec());
 }
 
 TEST_F(WebStateListTest, DetachWebStateAtIndexMiddle) {
   web_state_list_.InsertWebState(0, CreateWebState(kURL0), nullptr);
   web_state_list_.InsertWebState(1, CreateWebState(kURL1), nullptr);
   web_state_list_.InsertWebState(2, CreateWebState(kURL2), nullptr);
 
   // Sanity check before closing WebState.
   EXPECT_EQ(3, web_state_list_.count());
   EXPECT_EQ(kURL0, web_state_list_.GetWebStateAt(0)->GetVisibleURL().spec());
   EXPECT_EQ(kURL1, web_state_list_.GetWebStateAt(1)->GetVisibleURL().spec());
   EXPECT_EQ(kURL2, web_state_list_.GetWebStateAt(2)->GetVisibleURL().spec());
 
   observer_.ResetStatistics();
-  web_state_list_.DetachWebStateAt(1);
+  std::unique_ptr<web::WebState> old_web_state(
+      web_state_list_.DetachWebStateAt(1));
 
   EXPECT_TRUE(observer_.web_state_detached_called());
   EXPECT_EQ(2, web_state_list_.count());
   EXPECT_EQ(kURL0, web_state_list_.GetWebStateAt(0)->GetVisibleURL().spec());
   EXPECT_EQ(kURL2, web_state_list_.GetWebStateAt(1)->GetVisibleURL().spec());
 }
 
 TEST_F(WebStateListTest, DetachWebStateAtIndexLast) {
   web_state_list_.InsertWebState(0, CreateWebState(kURL0), nullptr);
   web_state_list_.InsertWebState(1, CreateWebState(kURL1), nullptr);
   web_state_list_.InsertWebState(2, CreateWebState(kURL2), nullptr);
 
   // Sanity check before closing WebState.
   EXPECT_EQ(3, web_state_list_.count());
   EXPECT_EQ(kURL0, web_state_list_.GetWebStateAt(0)->GetVisibleURL().spec());
   EXPECT_EQ(kURL1, web_state_list_.GetWebStateAt(1)->GetVisibleURL().spec());
   EXPECT_EQ(kURL2, web_state_list_.GetWebStateAt(2)->GetVisibleURL().spec());
 
   observer_.ResetStatistics();
-  web_state_list_.DetachWebStateAt(2);
+  std::unique_ptr<web::WebState> old_web_state(
+      web_state_list_.DetachWebStateAt(2));
 
   EXPECT_TRUE(observer_.web_state_detached_called());
   EXPECT_EQ(2, web_state_list_.count());
   EXPECT_EQ(kURL0, web_state_list_.GetWebStateAt(0)->GetVisibleURL().spec());
   EXPECT_EQ(kURL1, web_state_list_.GetWebStateAt(1)->GetVisibleURL().spec());
 }
 
 TEST_F(WebStateListTest, OwnershipBorrowed) {
   bool web_state_was_killed = false;
   auto test_web_state = base::MakeUnique<web::TestWebState>();
@@ skipping 142 matching lines @@
             web_state_list_.GetIndexOfLastWebStateOpenedBy(opener, start_index,
                                                            false));
 
   EXPECT_EQ(WebStateList::kInvalidIndex,
             web_state_list_.GetIndexOfNextWebStateOpenedBy(opener, start_index,
                                                            true));
   EXPECT_EQ(WebStateList::kInvalidIndex,
             web_state_list_.GetIndexOfLastWebStateOpenedBy(opener, start_index,
                                                            true));
 }