Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/metrics/tab_usage_recorder_web_state_list_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_CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_WEB_STATE_LIST_OBSERVER_H_
+#define IOS_CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_WEB_STATE_LIST_OBSERVER_H_
+
+#import "ios/shared/chrome/browser/tabs/web_state_list_observer.h"
+
+class TabUsageRecorder;
+
+// Informs the TabUsageRecorder that the active Tab has changed.
+class TabUsageRecorderWebStateListObserver : public WebStateListObserver {
+ public:
+  explicit TabUsageRecorderWebStateListObserver(
+      TabUsageRecorder* tab_usage_recorder);
+  ~TabUsageRecorderWebStateListObserver() override;
+
+ private:
+  // WebStateListObserver implementation.
+  void WebStateActivatedAt(WebStateList* web_state_list,
+                           web::WebState* old_web_state,
+                           web::WebState* new_web_state,
+                           int active_index,
+                           bool user_action) override;
+
+  TabUsageRecorder* tab_usage_recorder_;
+
+  DISALLOW_COPY_AND_ASSIGN(TabUsageRecorderWebStateListObserver);
+};
+
+#endif  // IOS_CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_WEB_STATE_LIST_OBSERVER_H_