Add a comment and DCHECK to require |session_storage| to be non-nil in WebState::CreateWithStorageS…

ios/web/web_state/web_state_impl.mm

 // Copyright 2013 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/web/web_state/web_state_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/logging.h"
@@ skipping 39 matching lines @@
   web_state->GetNavigationManagerImpl().InitializeSession();
 
   // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(web_state);
 }
 
 /* static */
 std::unique_ptr<WebState> WebState::CreateWithStorageSession(
     const CreateParams& params,
     CRWSessionStorage* session_storage) {
+  DCHECK(session_storage);
   std::unique_ptr<WebStateImpl> web_state(
       new WebStateImpl(params, session_storage));
 
   // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(web_state);
 }
 
 WebStateImpl::WebStateImpl(const CreateParams& params)
     : WebStateImpl(params, nullptr) {}
 
@@ skipping 716 matching lines @@
     const LoadCommittedDetails& load_details) {
   for (auto& observer : observers_)
     observer.NavigationItemCommitted(load_details);
 }
 
 WebState* WebStateImpl::GetWebState() {
   return this;
 }
 
 }  // namespace web