Implement 1:N mapping from ios::ChromeBrowserState to TabModel.

ios/chrome/browser/tabs/BUILD.gn

 # 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.
 
 source_set("tabs") {
   sources = [
     "tab.h",
     "tab_delegate.h",
     "tab_dialog_delegate.h",
     "tab_model.h",
+    "tab_model_list.h",
     "tab_model_observer.h",
     "tab_model_order_controller.h",
     "tab_model_synced_window_delegate.h",
     "tab_model_synced_window_delegate_getter.h",
     "tab_private.h",
     "tab_snapshotting_delegate.h",
   ]
   deps = [
     "//components/sessions",
     "//components/signin/ios/browser",
     "//components/sync_sessions",
     "//ios/net",
     "//ios/web",
     "//ui/base",
   ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("tabs_internal") {
   sources = [
     "tab.mm",
     "tab_model.mm",
     "tab_model_order_controller.mm",
     "tab_model_synced_window_delegate.mm",
     "tab_model_synced_window_delegate_getter.mm",
   ]
   deps = [
     ":tabs",
+    ":tabs_internal_arc",
     "//base",
     "//components/content_settings/core/browser",
     "//components/favicon/core",
     "//components/favicon/ios",
     "//components/google/core/browser",
     "//components/history/core/browser",
     "//components/history/ios/browser",
     "//components/infobars/core",
     "//components/keyed_service/core",
     "//components/metrics_services_manager",
@@ skipping 50 matching lines @@
     "//ios/chrome/browser/web",
     "//ios/chrome/browser/web:web_internal",
     "//ios/net",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/native_app_launcher",
     "//ios/web",
     "//net",
     "//ui/base",
     "//url",
   ]
+  allow_circular_includes_from = [ ":tabs_internal_arc" ]
   libs = [
     "CoreLocation.framework",
     "UIKit.framework",
   ]
 }
 
+source_set("tabs_internal_arc") {
+  sources = [
+    "tab_model_list.mm",
+  ]
+  deps = [
+    ":tabs",
+    "//base",
+    "//ios/chrome/browser/browser_state",
+  ]
+  libs = [ "Foundation.framework" ]
+  configs += [ "//build/config/compiler:enable_arc" ]
+}
+
 source_set("unit_tests") {
   testonly = true
   sources = [
     "tab_model_order_controller_unittest.mm",
     "tab_model_unittest.mm",
     "tab_unittest.mm",
   ]
   deps = [
     ":tabs",
     ":tabs_internal",
+    ":unit_tests_arc",
     "//base",
     "//components/bookmarks/test",
     "//components/history/core/browser",
     "//components/keyed_service/core",
     "//ios/chrome/browser",
     "//ios/chrome/browser/bookmarks",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/history",
     "//ios/chrome/browser/infobars",
     "//ios/chrome/browser/sessions",
     "//ios/chrome/browser/sessions:test_support",
     "//ios/chrome/browser/ui:ui_internal",
     "//ios/chrome/browser/web:web_internal",
     "//ios/chrome/test:test_support",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser:test_support",
     "//ios/public/provider/chrome/browser/native_app_launcher:test_support",
     "//ios/testing:ocmock_support",
     "//ios/web",
     "//ios/web:test_support",
     "//net",
     "//testing/gtest",
     "//third_party/ocmock",
   ]
 }
+
+source_set("unit_tests_arc") {
+  testonly = true
+  sources = [
+    "tab_model_list_unittest.mm",
+  ]
+  deps = [
+    ":tabs",
+    ":tabs_internal",
+    "//base",
+    "//ios/chrome/browser/browser_state:test_support",
+    "//ios/chrome/browser/sessions:test_support",
+    "//ios/web:test_support",
+    "//testing/gtest",
+  ]
+  configs += [ "//build/config/compiler:enable_arc" ]
+}