Convert some of TabModel logic to WebStateListObservers.

ios/chrome/browser/tabs/tab_parenting_observer.mm

+// 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.
+
+#import "ios/chrome/browser/tabs/tab_parenting_observer.h"
+
+#include "ios/chrome/browser/tab_parenting_global_observer.h"
+#import "ios/chrome/browser/tabs/legacy_tab_helper.h"
+#import "ios/chrome/browser/tabs/tab.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+namespace {
+void OnWebStateParented(web::WebState* web_state) {
+  TabParentingGlobalObserver::GetInstance()->OnTabParented(web_state);
+  Tab* tab = LegacyTabHelper::GetTabForWebState(web_state);
+  [tab fetchFavicon];
+}
+}  // namespace
+
+TabParentingObserver::TabParentingObserver() = default;
+
+TabParentingObserver::~TabParentingObserver() = default;
+
+void TabParentingObserver::WebStateInsertedAt(WebStateList* web_state_list,
+                                              web::WebState* web_state,
+                                              int index) {
+  OnWebStateParented(web_state);
+}
+
+void TabParentingObserver::WebStateMoved(WebStateList* web_state_list,
+                                         web::WebState* web_state,
+                                         int from_index,
+                                         int to_index) {}
+
+void TabParentingObserver::WebStateReplacedAt(WebStateList* web_state_list,
+                                              web::WebState* old_web_state,
+                                              web::WebState* new_web_state,
+                                              int index) {
+  OnWebStateParented(new_web_state);
+}
+
+void TabParentingObserver::WebStateDetachedAt(WebStateList* web_state_list,
+                                              web::WebState* web_state,
+                                              int index) {}