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

ios/chrome/browser/tabs/tab_model_unittest.mm

 // Copyright 2012 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 <objc/runtime.h>
 
 #include "base/mac/scoped_nsautorelease_pool.h"
 #include "base/memory/ptr_util.h"
 #include "base/run_loop.h"
 #include "base/strings/sys_string_conversions.h"
@@ skipping 129 matching lines @@
   TabModelTest() : web_client_(base::MakeUnique<ChromeWebClient>()) {}
 
  protected:
   void SetUp() override {
     DCHECK_CURRENTLY_ON(web::WebThread::UI);
     PlatformTest::SetUp();
 
     TestChromeBrowserState::Builder test_cbs_builder;
     chrome_browser_state_ = test_cbs_builder.Build();
 
-    sessionWindow_.reset([[SessionWindowIOS new] retain]);
+    sessionWindow_.reset([[SessionWindowIOS alloc] init]);

[sdefresne, 2017/01/10 17:49:28]

nit: this is unrelated but I based my tab_model_list_unittest.mm on this file
and saw this use of -new that is forbidden according to the style guide and
decided to remove it.

[marq (ping after 24h), 2017/01/11 15:56:47]

Acknowledged.

     // Create tab model with just a dummy session service so the async state
     // saving doesn't trigger unless actually wanted.
     base::scoped_nsobject<TestSessionService> test_service(
         [[TestSessionService alloc] init]);
     tabModel_.reset([[TabModel alloc]
         initWithSessionWindow:sessionWindow_.get()
                sessionService:test_service
                  browserState:chrome_browser_state_.get()]);
     [tabModel_ setWebUsageEnabled:YES];
     [tabModel_ setPrimary:YES];
@@ skipping 720 matching lines @@
                browserState:chrome_browser_state_.get()]);
   EXPECT_EQ(model.get().currentTab, [model tabAtIndex:1]);
   [model browserStateDestroyed];
 
   // Clean up.
   EXPECT_TRUE([[NSFileManager defaultManager] removeItemAtPath:stashPath
                                                          error:nullptr]);
 }
 
 }  // anonymous namespace