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

chrome/test/tab_switching/tab_switching_test.cc

Index: chrome/test/tab_switching/tab_switching_test.cc
diff --git a/chrome/test/tab_switching/tab_switching_test.cc b/chrome/test/tab_switching/tab_switching_test.cc
index 028642fd11f08c2428b1a7a70dc37e7120ff95f2..29776fdf73ffe999a84451fd4b927211fa157692 100644
--- a/chrome/test/tab_switching/tab_switching_test.cc
+++ b/chrome/test/tab_switching/tab_switching_test.cc
@@ -66,7 +66,7 @@ class TabSwitchingUITest : public UIPerfTest {
     bool important) {
       std::string times;
       for (int i = 0; i < kNumCycles; ++i)
-        StringAppendF(&times, "%.2f,", timings[i].InMillisecondsF());
+        base::StringAppendF(&times, "%.2f,", timings[i].InMillisecondsF());
       PrintResultList("times", "", label, times, "ms", important);
   }