base: Get rid of 'using' declaration of StringAppendF.

chrome/browser/metrics/metrics_log_unittest.cc

 // Copyright (c) 2006-2010 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 <string>
 
 #include "base/string_util.h"
 #include "base/time.h"
 #include "chrome/browser/metrics/metrics_log.h"
 #include "googleurl/src/gurl.h"
@@ skipping 195 matching lines @@
   };
 
   for (size_t i = 0; i < ARRAYSIZE_UNSAFE(cases); i++) {
     std::string hash_string = MetricsLog::CreateHash(cases[i].input);
 
     // Convert to hex string
     // We're only checking the first 8 bytes, because that's what
     // the metrics server uses.
     std::string hash_hex = "0x";
     for (size_t j = 0; j < 8; j++) {
-      StringAppendF(&hash_hex, "%02x",
-                    static_cast<uint8>(hash_string.data()[j]));
+      base::StringAppendF(&hash_hex, "%02x",
+                          static_cast<uint8>(hash_string.data()[j]));
     }
     EXPECT_EQ(cases[i].output, hash_hex);
   }
 };