Introduce BrowserList and Browser classes for iOS.

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"
 #import "ios/web/public/test/fakes/test_navigation_manager.h"
 #import "ios/web/public/test/fakes/test_web_state.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/platform_test.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 const char kURL0[] = "https://chromium.org/0";
 const char kURL1[] = "https://chromium.org/1";
 const char kURL2[] = "https://chromium.org/2";
 const char kSupportsUserDataDeathGuardKey = '\0';
 
 // A base::SupportsUserData::Data that tracks whether a base::SupportsUserData
 // has been deleted (the fact is recorded in a provided pointer as part of the
 // object destruction).
 class SupportsUserDataDeathGuard : public base::SupportsUserData::Data {
@@ skipping 501 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));
 }