Convert some of TabModel logic to WebStateListObservers.

ios/chrome/browser/tabs/tab_model_observers_bridge.h

Index: ios/chrome/browser/tabs/tab_model_observers_bridge.h
diff --git a/ios/chrome/browser/tabs/tab_model_observers_bridge.h b/ios/chrome/browser/tabs/tab_model_observers_bridge.h
new file mode 100644
index 0000000000000000000000000000000000000000..d1f7242f410e311de1f7b2bf1321f012f6f48ca5
--- /dev/null
+++ b/ios/chrome/browser/tabs/tab_model_observers_bridge.h
@@ -0,0 +1,24 @@
+// 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_BRIDGE_H_
+#define IOS_CHROME_BROWSER_TABS_TAB_MODEL_OBSERVERS_BRIDGE_H_
+
+#import "ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h"
+
+@class TabModel;
+@class TabModelObservers;
+
+// Bridge WebStateListObserver events to TabModelObservers.
+@interface TabModelObserversBridge : NSObject<WebStateListObserving>
+
+- (instancetype)initWithTabModel:(TabModel*)tabModel
+               tabModelObservers:(TabModelObservers*)tabModelObservers
+    NS_DESIGNATED_INITIALIZER;
+
+- (instancetype)init NS_UNAVAILABLE;
+
+@end
+
+#endif  // IOS_CHROME_BROWSER_TABS_TAB_MODEL_OBSERVERS_BRIDGE_H_