Revert of Move JoinString to the base namespace

chrome/test/base/web_ui_browser_test.cc

Index: chrome/test/base/web_ui_browser_test.cc
diff --git a/chrome/test/base/web_ui_browser_test.cc b/chrome/test/base/web_ui_browser_test.cc
index 0b9d3a5bf98c8b3407b435d6a5eecd0c2139764c..4ba0b0b0006c87a9a0c38f4c67b2157979b46dba 100644
--- a/chrome/test/base/web_ui_browser_test.cc
+++ b/chrome/test/base/web_ui_browser_test.cc
@@ -11,7 +11,6 @@
 #include "base/memory/ref_counted_memory.h"
 #include "base/path_service.h"
 #include "base/strings/string_util.h"
-#include "base/strings/utf_string_conversions.h"
 #include "base/values.h"
 #include "chrome/browser/chrome_content_browser_client.h"
 #include "chrome/browser/profiles/profile.h"
@@ -419,7 +418,7 @@
   if (!libraries_preloaded_) {
     BuildJavascriptLibraries(&libraries);
     if (!preload_host) {
-      content = base::JoinString(libraries, base::ASCIIToUTF16("\n"));
+      content = JoinString(libraries, '\n');
       libraries.clear();
     }
   }