Convert some of TabModel logic to WebStateListObservers.

ios/chrome/browser/tabs/tab_parenting_observer.h

Index: ios/chrome/browser/tabs/tab_parenting_observer.h
diff --git a/ios/chrome/browser/tabs/tab_parenting_observer.h b/ios/chrome/browser/tabs/tab_parenting_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..acb47dc860a5519acd478125552e9fe3bd44dd75
--- /dev/null
+++ b/ios/chrome/browser/tabs/tab_parenting_observer.h
@@ -0,0 +1,36 @@
+// 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_PARENTING_OBSERVER_H_
+#define IOS_CHROME_BROWSER_TABS_TAB_PARENTING_OBSERVER_H_
+
+#include "base/macros.h"
+#import "ios/shared/chrome/browser/tabs/web_state_list_observer.h"
+
+class TabParentingObserver : public WebStateListObserver {
+ public:
+  TabParentingObserver();
+  ~TabParentingObserver() 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(TabParentingObserver);
+};
+
+#endif  // IOS_CHROME_BROWSER_TABS_TAB_PARENTING_OBSERVER_H_