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