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

ios/chrome/browser/tabs/tab_model_unittest.mm

Index: ios/chrome/browser/tabs/tab_model_unittest.mm
diff --git a/ios/chrome/browser/tabs/tab_model_unittest.mm b/ios/chrome/browser/tabs/tab_model_unittest.mm
index d99a886419c6e5a8cdf66fcf18b1c232b2c8b296..4d2e906842ff27a399bc0dd96a3403e288a91727 100644
--- a/ios/chrome/browser/tabs/tab_model_unittest.mm
+++ b/ios/chrome/browser/tabs/tab_model_unittest.mm
@@ -147,7 +147,7 @@ class TabModelTest : public PlatformTest {
     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(