Move JoinString to the base namespace

tools/set_default_handler/set_default_handler_main.cc

Index: tools/set_default_handler/set_default_handler_main.cc
diff --git a/tools/set_default_handler/set_default_handler_main.cc b/tools/set_default_handler/set_default_handler_main.cc
index 8b4b343eefc9c50f0db28837e8624450f1022604..92edb6c11825afa053489fa48501e8eea397eba4 100644
--- a/tools/set_default_handler/set_default_handler_main.cc
+++ b/tools/set_default_handler/set_default_handler_main.cc
@@ -56,7 +56,7 @@ int wmain(int argc, wchar_t* argv[]) {
     printf("success\n");
   } else if (!choices.empty()) {
     printf("failed to set program. possible choices: %ls\n",
-           JoinString(choices, L", ").c_str());
+           base::JoinString(choices, L", ").c_str());
   } else {
     printf("failed with HRESULT: %0x08X\n", result);
   }