[ios] Add a delegate to WebStateList class.

ios/clean/chrome/browser/model/browser_web_state_list_delegate.mm

Index: ios/clean/chrome/browser/model/browser_web_state_list_delegate.mm
diff --git a/ios/clean/chrome/browser/model/browser_web_state_list_delegate.mm b/ios/clean/chrome/browser/model/browser_web_state_list_delegate.mm
new file mode 100644
index 0000000000000000000000000000000000000000..4f96ede1dbdb598f415e527e5136ea9fbe05ac0b
--- /dev/null
+++ b/ios/clean/chrome/browser/model/browser_web_state_list_delegate.mm
@@ -0,0 +1,20 @@
+// 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/clean/chrome/browser/model/browser_web_state_list_delegate.h"
+
+#include "base/logging.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+BrowserWebStateListDelegate::BrowserWebStateListDelegate(Browser* browser)
+    : browser_(browser) {
+  DCHECK(browser_);
+}
+
+BrowserWebStateListDelegate::~BrowserWebStateListDelegate() = default;
+
+void BrowserWebStateListDelegate::WillAddWebState(web::WebState* web_state) {}