Extract TabModelObservers from tab_model.mm to its own file.

ios/chrome/browser/tabs/tab_model_observers.h

Index: ios/chrome/browser/tabs/tab_model_observers.h
diff --git a/ios/chrome/browser/tabs/tab_model_observers.h b/ios/chrome/browser/tabs/tab_model_observers.h
new file mode 100644
index 0000000000000000000000000000000000000000..ab382a9c9f3179ebde14b044833230cbfb86321f
--- /dev/null
+++ b/ios/chrome/browser/tabs/tab_model_observers.h
@@ -0,0 +1,17 @@
+// 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.
+
+#ifndef IOS_CHROME_BROWSER_TABS_TAB_MODEL_OBSERVERS_H_
+#define IOS_CHROME_BROWSER_TABS_TAB_MODEL_OBSERVERS_H_
+
+#import "base/ios/crb_protocol_observers.h"
+#import "ios/chrome/browser/tabs/tab_model_observer.h"
+
+@interface TabModelObservers : CRBProtocolObservers<TabModelObserver>
+
++ (instancetype)observers;
+
+@end
+
+#endif  // IOS_CHROME_BROWSER_TABS_TAB_MODEL_OBSERVERS_H_