Use a direct include of strings headers in base/.

base/metrics/sparse_histogram.cc

 // Copyright (c) 2012 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 "base/metrics/sparse_histogram.h"
 
 #include "base/metrics/sample_map.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/pickle.h"
-#include "base/stringprintf.h"
+#include "base/strings/stringprintf.h"
 #include "base/synchronization/lock.h"
 
 using std::map;
 using std::string;
 
 namespace base {
 
 typedef HistogramBase::Count Count;
 typedef HistogramBase::Sample Sample;
 
@@ skipping 148 matching lines @@
                                        std::string* output) const {
   StringAppendF(output,
                 "Histogram: %s recorded %d samples",
                 histogram_name().c_str(),
                 total_count);
   if (flags() & ~kHexRangePrintingFlag)
     StringAppendF(output, " (flags = 0x%x)", flags() & ~kHexRangePrintingFlag);
 }
 
 }  // namespace base