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

cpp/CMakeLists.txt

@@ -175,7 +175,7 @@ endif ()
 
 set (
   SOURCES
-  "src/base/string_piece.cc"
+  "src/base/strings/string_piece.cc"
   "src/phonenumbers/asyoutypeformatter.cc"
   "src/phonenumbers/default_logger.cc"
   "src/phonenumbers/logger.cc"

cpp/src/phonenumbers/utf/stringpiece.h

@@ -16,8 +16,7 @@
 #ifndef STRINGS_STRINGPIECE_H_
 #define STRINGS_STRINGPIECE_H_
 
-//#include "third_party/chromium/src/base/string_piece.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 
 using base::StringPiece;