Use a direct include of strings headers in chromeos/.

chromeos/dbus/volume_state.cc

 // Copyright (c) 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 "chromeos/dbus/volume_state.h"
 
 #include "base/format_macros.h"
-#include "base/stringprintf.h"
+#include "base/strings/stringprintf.h"
 
 namespace chromeos {
 
 VolumeState::VolumeState()
     : output_volume(0),
       output_mute(false),
       input_gain(0),
       input_mute(false) {
 }
 
@@ skipping 12 matching lines @@
                       "input_gain = %d ",
                       input_gain);
   base::StringAppendF(&result,
                       "input_mute = %s ",
                       input_mute ? "true" : "false");
 
   return result;
 }
 
 }  // namespace chromeos