Introduce BrowserList and Browser classes for iOS.

ios/clean/chrome/browser/model/browser.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/clean/chrome/browser/model/browser.h"
+
+#include "base/logging.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+Browser::Browser(ios::ChromeBrowserState* browser_state)
+    : web_state_list_(WebStateList::WebStateOwned),
+      browser_state_(browser_state) {
+  DCHECK(browser_state_);
+}
+
+Browser::~Browser() = default;