Convert some of TabModel logic to WebStateListObservers.

ios/chrome/browser/tabs/tab_model_tab_parenting_observer.mm

Index: ios/chrome/browser/tabs/tab_model_tab_parenting_observer.mm
diff --git a/ios/chrome/browser/tabs/tab_model_tab_parenting_observer.mm b/ios/chrome/browser/tabs/tab_model_tab_parenting_observer.mm
new file mode 100644
index 0000000000000000000000000000000000000000..8f85c73c89dfb7c1e747c54e13ec34bc0c35aff3
--- /dev/null
+++ b/ios/chrome/browser/tabs/tab_model_tab_parenting_observer.mm
@@ -0,0 +1,40 @@
+// 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_model_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
+
+@implementation TabModelTabParentingObserver
+
+#pragma mark WebStateListObserver
+
+- (void)webStateList:(WebStateList*)webStateList
+    webStateInserted:(web::WebState*)webState
+             atIndex:(NSUInteger)index {
+  [self parentedWebState:webState];
+}
+
+- (void)webStateList:(WebStateList*)webStateList
+    webStateReplaced:(web::WebState*)oldWebState
+          byWebState:(web::WebState*)newWebState
+             atIndex:(NSUInteger)index {
+  [self parentedWebState:newWebState];
+}
+
+#pragma mark Private methods
+
+- (void)parentedWebState:(web::WebState*)webState {
+  TabParentingGlobalObserver::GetInstance()->OnTabParented(webState);
+  Tab* tab = LegacyTabHelper::GetTabForWebState(webState);
+  [tab fetchFavicon];
+}
+
+@end