Move string_piece.h to base/strings.

base/base.gypi

 # 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.
 
 {
   'target_defaults': {
     'variables': {
       'base_target': 0,
     },
     'target_conditions': [
@@ skipping 372 matching lines @@
           'sha1.h',
           'sha1_portable.cc',
           'sha1_win.cc',
           'shared_memory.h',
           'shared_memory_android.cc',
           'shared_memory_nacl.cc',
           'shared_memory_posix.cc',
           'shared_memory_win.cc',
           'single_thread_task_runner.h',
           'stl_util.h',
-          'string_piece.cc',
-          'string_piece.h',
           'string_util.cc',
           'string_util.h',
           'string_util_posix.h',
           'string_util_win.h',
           'string16.cc',
           'string16.h',
           'stringprintf.cc',
           'stringprintf.h',
           'strings/string_split.cc',
           'strings/string_split.h',
           'strings/string_number_conversions.cc',
           'strings/string_number_conversions.h',
+          'strings/string_piece.cc',
+          'strings/string_piece.h',
           'strings/string_tokenizer.h',
           'strings/stringize_macros.h',
           'strings/sys_string_conversions.h',
           'strings/sys_string_conversions_mac.mm',
           'strings/sys_string_conversions_posix.cc',
           'strings/sys_string_conversions_win.cc',
           'strings/utf_offset_string_conversions.cc',
           'strings/utf_offset_string_conversions.h',
           'strings/utf_string_conversion_utils.cc',
           'strings/utf_string_conversion_utils.h',
@@ skipping 347 matching lines @@
           ['<(use_system_nspr)==1 and >(nacl_untrusted_build)==0', {
             'sources/': [
               ['exclude', '^third_party/nspr/'],
             ],
           }],
         ],
       }],
     ],
   },
 }