Move string_piece.h to base/strings.

base/base.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'includes': [
     '../build/win_precompile.gypi',
@@ skipping 544 matching lines @@
         'safe_numerics_unittest.nc',
         'scoped_native_library_unittest.cc',
         'scoped_observer.h',
         'security_unittest.cc',
         'sequence_checker_unittest.cc',
         'sequence_checker_impl_unittest.cc',
         'sha1_unittest.cc',
         'shared_memory_unittest.cc',
         'stl_util_unittest.cc',
         'string16_unittest.cc',
-        'string_piece_unittest.cc',
         'string_util_unittest.cc',
         'stringprintf_unittest.cc',
         'strings/string_number_conversions_unittest.cc',
+        'strings/string_piece_unittest.cc',
         'strings/string_split_unittest.cc',
         'strings/string_tokenizer_unittest.cc',
         'strings/stringize_macros_unittest.cc',
         'strings/sys_string_conversions_mac_unittest.mm',
         'strings/sys_string_conversions_unittest.cc',
         'strings/utf_offset_string_conversions_unittest.cc',
         'synchronization/cancellation_flag_unittest.cc',
         'synchronization/condition_variable_unittest.cc',
         'synchronization/lock_unittest.cc',
         'synchronization/waitable_event_unittest.cc',
@@ skipping 641 matching lines @@
             'base_unittests.isolate',
           ],
           'sources': [
             'base_unittests.isolate',
           ],
         },
       ],
     }],
   ],
 }