[ios] WebStateList owns all WebState it manages.

ios/chrome/browser/tabs/legacy_tab_helper.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/legacy_tab_helper.h"
 
 #import "ios/chrome/browser/tabs/tab.h"
+#import "ios/chrome/browser/tabs/tab_private.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 DEFINE_WEB_STATE_USER_DATA_KEY(LegacyTabHelper);
 
 // static
-void LegacyTabHelper::CreateForWebState(web::WebState* web_state, Tab* tab) {
+void LegacyTabHelper::CreateForWebState(web::WebState* web_state) {
   DCHECK(web_state);
   DCHECK(!FromWebState(web_state));
-  web_state->SetUserData(UserDataKey(), new LegacyTabHelper(web_state, tab));
+  web_state->SetUserData(UserDataKey(),
+                         std::unique_ptr<base::SupportsUserData::Data>(
+                             new LegacyTabHelper(web_state)));
 }
 
 // static
 Tab* LegacyTabHelper::GetTabForWebState(web::WebState* web_state) {
   DCHECK(web_state);
   LegacyTabHelper* tab_helper = LegacyTabHelper::FromWebState(web_state);
   return tab_helper ? tab_helper->tab_.get() : nil;
 }
 
 LegacyTabHelper::~LegacyTabHelper() = default;
 
-LegacyTabHelper::LegacyTabHelper(web::WebState* web_state, Tab* tab)
-    : tab_(tab) {
-  DCHECK_EQ(web_state, tab.webState);
+LegacyTabHelper::LegacyTabHelper(web::WebState* web_state)
+    : WebStateObserver(web_state),
+      tab_([[Tab alloc] initWithWebState:web_state]) {}
+
+void LegacyTabHelper::WebStateDestroyed() {
+  [tab_ onWebStateDestroyed];

[rohitrao (ping after 24h), 2017/03/26 23:21:21]

Is this better than making Tab itself a WebStateObserver?

[sdefresne, 2017/03/28 15:15:05]

Tab is already a CRWWebstateObserver. I agree that only one of the two classes
need to be an observer (though I would have preferred the tab helper to be the
observer as most of the tab helpers are WebStateObserver in addition to being
WebStateUserData).

TL;DR: Done.

 }