Use a direct include of strings headers in chrome/test/.

chrome/test/perf/mach_ports_test.cc

 // Copyright (c) 2012 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 <vector>
 
 #include "base/memory/ref_counted.h"
-#include "base/string_util.h"
+#include "base/strings/string_util.h"
 #include "chrome/common/automation_constants.h"
 #include "chrome/common/automation_messages.h"
 #include "chrome/test/automation/automation_proxy.h"
 #include "chrome/test/automation/browser_proxy.h"
 #include "chrome/test/automation/tab_proxy.h"
 #include "chrome/test/perf/perf_test.h"
 #include "chrome/test/ui/ui_perf_test.h"
 #include "net/test/spawned_test_server/spawned_test_server.h"
 
 namespace {
@@ skipping 74 matching lines @@
   for (int i = 0; i < tab_count - 1; ++i) {
     scoped_refptr<TabProxy> tab(browser->GetActiveTab());
     ASSERT_TRUE(tab.get());
 
     EXPECT_TRUE(tab->Close(true));
     RecordPortCount();
   }
 }
 
 }  // namespace