libchromeos: Migrate string_utils from Buffet to libchromeos

BUG=chromium:405714
TEST=USE=buffet ./build_packages

Change-Id: I95d8977aed6ffd718751de058d4cdda3a5395c25
Reviewed-on: https://chromium-review.googlesource.com/213361
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/device_registration_info.cc b/buffet/device_registration_info.cc
index 800cfa0..b111de1 100644
--- a/buffet/device_registration_info.cc
+++ b/buffet/device_registration_info.cc
@@ -10,6 +10,7 @@
 
 #include <base/json/json_writer.h>
 #include <base/values.h>
+#include <chromeos/string_utils.h>
 
 #include "buffet/commands/command_definition.h"
 #include "buffet/commands/command_manager.h"
@@ -19,7 +20,6 @@
 #include "buffet/http_utils.h"
 #include "buffet/mime_utils.h"
 #include "buffet/storage_impls.h"
-#include "buffet/string_utils.h"
 #include "buffet/url_utils.h"
 
 const char buffet::kErrorDomainOAuth2[] = "oauth2";
@@ -67,7 +67,7 @@
     const std::string& access_token_type,
     const std::string& access_token) {
   std::string authorization =
-      buffet::string_utils::Join(' ', access_token_type, access_token);
+      chromeos::string_utils::Join(' ', access_token_type, access_token);
   return {buffet::http::request_header::kAuthorization, authorization};
 }