[ObjC ARC] Converts ios/chrome/browser:unit_tests to ARC.

ios/chrome/browser/install_time_util_unittest.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.
 
 #include "ios/chrome/browser/install_time_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/gtest_mac.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 TEST(InstallTimeUtilTest, ComputeInstallationTime) {
   const base::Time null_time = base::Time();
   const base::Time now = base::Time::Now();
   const base::Time one_month_ago = now - base::TimeDelta::FromDays(30);
   const base::Time sentinel =
       base::Time::FromTimeT(install_time_util::kUnknownInstallDate);
 
   base::Time install_time;
   base::TimeDelta delta_from_now;
 
@@ skipping 19 matching lines @@
   EXPECT_FALSE(install_time.is_null());
   EXPECT_EQ(sentinel, install_time);
 
   // Case 4: Not first run, and NSUserDefaults had an install time.  Should
   // migrate that to LocalState.
   install_time =
       install_time_util::ComputeInstallationTimeInternal(false, one_month_ago);
   EXPECT_FALSE(install_time.is_null());
   EXPECT_EQ(one_month_ago, install_time);
 }