Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/tabs/tab_model_selected_tab_observer.h

Index: ios/chrome/browser/tabs/tab_model_selected_tab_observer.h
diff --git a/ios/chrome/browser/tabs/tab_model_selected_tab_observer.h b/ios/chrome/browser/tabs/tab_model_selected_tab_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..f4f53d0331d07b12a0ecb8d7273a97573d68fb55
--- /dev/null
+++ b/ios/chrome/browser/tabs/tab_model_selected_tab_observer.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_SELECTED_TAB_OBSERVER_H_
+#define IOS_CHROME_BROWSER_TABS_TAB_MODEL_SELECTED_TAB_OBSERVER_H_
+
+#import <Foundation/Foundation.h>
+
+#import "ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h"
+
+@class TabModel;
+
+// Listen to WebStateList active WebState change events and then save the
+// old Tab state, fire notification and save the session if necessary.
+@interface TabModelSelectedTabObserver : NSObject<WebStateListObserving>
+
+- (instancetype)initWithTabModel:(TabModel*)tabModel NS_DESIGNATED_INITIALIZER;
+
+- (instancetype)init NS_UNAVAILABLE;
+
+@end
+
+#endif  // IOS_CHROME_BROWSER_TABS_TAB_MODEL_SELECTED_TAB_OBSERVER_H_