Revert of Move JoinString to the base namespace

ash/system/chromeos/tray_display_unittest.cc

 // Copyright 2013 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 "ash/system/chromeos/tray_display.h"
 
 #include "ash/display/display_manager.h"
 #include "ash/root_window_controller.h"
 #include "ash/screen_util.h"
 #include "ash/shell.h"
@@ skipping 27 matching lines @@
   } else {
     lines.push_back(l10n_util::GetStringFUTF16(
         IDS_ASH_STATUS_TRAY_DISPLAY_SINGLE_DISPLAY,
         name1, base::UTF8ToUTF16(data1)));
   }
   if (!name2.empty()) {
     lines.push_back(l10n_util::GetStringFUTF16(
         IDS_ASH_STATUS_TRAY_DISPLAY_SINGLE_DISPLAY,
         name2, base::UTF8ToUTF16(data2)));
   }
-  return base::JoinString(lines, base::ASCIIToUTF16("\n"));
+  return JoinString(lines, '\n');
 }
 
 base::string16 GetMirroredTooltipText(const base::string16& headline,
                                       const base::string16& name,
                                       const std::string& data) {
   return GetTooltipText(headline, name, data, base::string16(), "");
 }
 
 base::string16 GetFirstDisplayName() {
   DisplayManager* display_manager = Shell::GetInstance()->display_manager();
@@ skipping 530 matching lines @@
   // rotate the second.
   UpdateDisplay("400x400,200x200/r");
   EXPECT_EQ(l10n_util::GetStringFUTF16(
                 IDS_ASH_STATUS_TRAY_DISPLAY_ROTATED, GetSecondDisplayName(),
                 l10n_util::GetStringUTF16(
                     IDS_ASH_STATUS_TRAY_DISPLAY_ORIENTATION_90)),
             GetDisplayNotificationAdditionalText());
 }
 
 }  // namespace ash