Convert some of TabModel logic to WebStateListObservers.

ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.h

+// 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_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_METRICS_OBSERVER_H_
+#define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_METRICS_OBSERVER_H_
+
+#include "base/macros.h"
+#import "ios/shared/chrome/browser/tabs/web_state_list_observer.h"
+
+class WebStateListMetricsObserver : public WebStateListObserver {
+ public:
+  WebStateListMetricsObserver();
+  ~WebStateListMetricsObserver() override;
+
+  // WebStateListObserver implementation.
+  void WebStateInsertedAt(WebStateList* web_state_list,
+                          web::WebState* web_state,
+                          int index) override;
+  void WebStateMoved(WebStateList* web_state_list,
+                     web::WebState* web_state,
+                     int from_index,
+                     int to_index) override;
+  void WebStateReplacedAt(WebStateList* web_state_list,
+                          web::WebState* old_web_state,
+                          web::WebState* new_web_state,
+                          int index) override;
+  void WebStateDetachedAt(WebStateList* web_state_list,
+                          web::WebState* web_state,
+                          int index) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(WebStateListMetricsObserver);
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_METRICS_OBSERVER_H_