[ObjC ARC] Converts ios/web:ios_web_public_unittests to ARC.

ios/web/public/user_agent_unittest.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/web/public/user_agent.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 // Tests conversions between UserAgentType values and their descriptions
 TEST(UserAgentTest, UserAgentTypeDescription) {
   const std::string kMobileDescription("MOBILE");
   const std::string kDesktopDescription("DESKTOP");
   const std::string kNoneDescription("NONE");
   const std::string kInvalidDescription(
       "not returned by GetUserAgentTypeDescription()");
   EXPECT_EQ(kMobileDescription,
             GetUserAgentTypeDescription(UserAgentType::MOBILE));
   EXPECT_EQ(kDesktopDescription,
             GetUserAgentTypeDescription(UserAgentType::DESKTOP));
   EXPECT_EQ(kNoneDescription, GetUserAgentTypeDescription(UserAgentType::NONE));
   EXPECT_EQ(UserAgentType::MOBILE,
             GetUserAgentTypeWithDescription(kMobileDescription));
   EXPECT_EQ(UserAgentType::DESKTOP,
             GetUserAgentTypeWithDescription(kDesktopDescription));
   EXPECT_EQ(UserAgentType::NONE,
             GetUserAgentTypeWithDescription(kNoneDescription));
   EXPECT_EQ(UserAgentType::NONE,
             GetUserAgentTypeWithDescription(kInvalidDescription));
 }
 
 }  // namespace web