Move stringpiece.h and stringprintf.h to base/.

Change-Id: I7f71b4a12f99c5f81771146c66629ae5a947b229
diff --git a/src/base/unix_file/string_file.h b/src/base/unix_file/string_file.h
index 4277dc2..8944373 100644
--- a/src/base/unix_file/string_file.h
+++ b/src/base/unix_file/string_file.h
@@ -22,8 +22,8 @@
 #include <string>
 
 #include "base/macros.h"
+#include "base/stringpiece.h"
 #include "base/unix_file/random_access_file.h"
-#include "stringpiece.h"
 
 namespace unix_file {