Move two generic string split functions from sync API to their own API in base/string_split.

base/base.gypi

 # Copyright (c) 2009 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 177 matching lines @@
           'simple_thread.h',
           'singleton.h',
           'spin_wait.h',
           'stack_container.h',
           'stats_counters.h',
           'stats_table.cc',
           'stats_table.h',
           'stl_util-inl.h',
           'string_piece.cc',
           'string_piece.h',
+          'string_split.cc',
+          'string_split.h'
           'string_tokenizer.h',
           'string_util.cc',
           'string_util.h',
           'string_util_win.h',
           'sys_info.h',
           'sys_info_chromeos.cc',
           'sys_info_freebsd.cc',
           'sys_info_linux.cc',
           'sys_info_mac.cc',
           'sys_info_openbsd.cc',
@@ skipping 380 matching lines @@
           'configurations': {
             'Common': {
               'msvs_target_platform': 'x64',
             },
           },
         },
       ],
     }],
   ],
 }