CPP: Move base/string_piece.* to base/strings/.

This change is needed to embed libphonenumber into Chromium past r191206.

BUG=http://crbug.com/236272
R=jia.shao.peng@gmail.com

Review URL: https://codereview.appspot.com/9014043

git-svn-id: http://libphonenumber.googlecode.com/svn/trunk/cpp/src/phonenumbers@566 ee073f10-1060-11df-b6a4-87a95322a99c
1 file changed