Move string files in base/ to the string subdirectory.

base/base.gypi

Index: base/base.gypi
diff --git a/base/base.gypi b/base/base.gypi
index 456bcb3b10136f3b64882f2af54d1fba12928c3a..c9faa43946b9d23ef1b5bf9c074e230f4cb9f86b 100644
--- a/base/base.gypi
+++ b/base/base.gypi
@@ -423,17 +423,10 @@
           'sha1_win.cc',
           'single_thread_task_runner.h',
           'stl_util.h',
-          'string_util.cc',
-          'string_util.h',
-          'string_util_constants.cc',
-          'string_util_posix.h',
-          'string_util_win.h',
-          'string16.cc',
-          'string16.h',
-          'stringprintf.cc',
-          'stringprintf.h',
           'strings/latin1_string_conversions.cc',
           'strings/latin1_string_conversions.h',
+          'strings/string16.cc',
+          'strings/string16.h',
           'strings/string_number_conversions.cc',
           'strings/string_split.cc',
           'strings/string_split.h',
@@ -441,7 +434,14 @@
           'strings/string_piece.cc',
           'strings/string_piece.h',
           'strings/string_tokenizer.h',
+          'strings/string_util.cc',
+          'strings/string_util.h',
+          'strings/string_util_constants.cc',
+          'strings/string_util_posix.h',
+          'strings/string_util_win.h',
           'strings/stringize_macros.h',
+          'strings/stringprintf.cc',
+          'strings/stringprintf.h',
           'strings/sys_string_conversions.h',
           'strings/sys_string_conversions_mac.mm',
           'strings/sys_string_conversions_posix.cc',
@@ -785,7 +785,7 @@
               # Not using sha1_win.cc because it may have caused a
               # regression to page cycler moz.
               'sha1_win.cc',
-              'string16.cc',
+              'strings/string16.cc',
             ],
           },],
           ['<(use_ozone) == 1', {