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

chrome/browser/metrics/metrics_log_unittest.cc

Index: chrome/browser/metrics/metrics_log_unittest.cc
diff --git a/chrome/browser/metrics/metrics_log_unittest.cc b/chrome/browser/metrics/metrics_log_unittest.cc
index 27c84c6ab1ecc608a6f9b92f35caf6122948ef19..184500d237ebe18481e3116d402d5c70293e9e1e 100644
--- a/chrome/browser/metrics/metrics_log_unittest.cc
+++ b/chrome/browser/metrics/metrics_log_unittest.cc
@@ -213,8 +213,8 @@ TEST(MetricsLogTest, CreateHash) {
     // 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);
   }