Move the iOS user agent functions to ios/web/public.

ios/web/public/user_agent.h

+// Copyright 2014 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.
+
+#ifndef IOS_WEB_PUBLIC_USER_AGENT_H_
+#define IOS_WEB_PUBLIC_USER_AGENT_H_
+
+#include <string>
+
+namespace web {
+
+// Returns the user agent to use for the given product name.
+// The returned user agent is very similar to that used by Mobile Safari, for
+// web page compatibility.
+std::string BuildUserAgentFromProduct(const std::string& product);
+
+}  // namespace web
+
+#endif  // IOS_WEB_PUBLIC_USER_AGENT_H_