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

ios/chrome/browser/tabs/tab_model_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/chrome/browser/tabs/tab_model_list.h"
+
+#import "base/mac/scoped_nsobject.h"
+#include "base/memory/ptr_util.h"
+#include "ios/chrome/browser/browser_state/test_chrome_browser_state.h"
+#import "ios/chrome/browser/sessions/test_session_service.h"
+#import "ios/chrome/browser/tabs/tab_model.h"
+#include "ios/web/public/test/test_web_thread_bundle.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
+
+class TabModelListTest : public PlatformTest {
+ public:
+  TabModelListTest() {
+    TestChromeBrowserState::Builder test_cbs_builder;
+    chrome_browser_state_ = test_cbs_builder.Build();
+  }
+
+  TabModel* CreateTabModel() {
+    return [[TabModel alloc]
+        initWithSessionWindow:nil
+               sessionService:[[TestSessionService alloc] init]
+                 browserState:chrome_browser_state_.get()];
+  }
+
+  NSArray<TabModel*>* RegisteredTabModels() {
+    return GetTabModelsForChromeBrowserState(chrome_browser_state_.get());
+  }
+
+ private:
+  web::TestWebThreadBundle thread_bundle_;
+  std::unique_ptr<TestChromeBrowserState> chrome_browser_state_;
+
+  DISALLOW_COPY_AND_ASSIGN(TabModelListTest);
+};
+
+TEST_F(TabModelListTest, RegisterAndUnregister) {
+  EXPECT_EQ([RegisteredTabModels() count], 0u);
+
+  TabModel* tab_model = CreateTabModel();
+  EXPECT_EQ([RegisteredTabModels() count], 1u);
+  EXPECT_NE([RegisteredTabModels() indexOfObject:tab_model],
+            static_cast<NSUInteger>(NSNotFound));
+
+  [tab_model browserStateDestroyed];
+
+  EXPECT_EQ([RegisteredTabModels() count], 0u);
+}
+
+TEST_F(TabModelListTest, SupportsMultipleTabModel) {
+  EXPECT_EQ([RegisteredTabModels() count], 0u);
+
+  TabModel* tab_model1 = CreateTabModel();
+  EXPECT_EQ([RegisteredTabModels() count], 1u);
+  EXPECT_NE([RegisteredTabModels() indexOfObject:tab_model1],
+            static_cast<NSUInteger>(NSNotFound));
+
+  TabModel* tab_model2 = CreateTabModel();
+  EXPECT_EQ([RegisteredTabModels() count], 2u);
+  EXPECT_NE([RegisteredTabModels() indexOfObject:tab_model2],
+            static_cast<NSUInteger>(NSNotFound));
+
+  [tab_model1 browserStateDestroyed];
+  [tab_model2 browserStateDestroyed];
+
+  EXPECT_EQ([RegisteredTabModels() count], 0u);
+}