Move more string_util functions to base namespace.

extensions/browser/api/networking_config/networking_config_service.cc

Index: extensions/browser/api/networking_config/networking_config_service.cc
diff --git a/extensions/browser/api/networking_config/networking_config_service.cc b/extensions/browser/api/networking_config/networking_config_service.cc
index b10813dcc41638107c59a5b6c606eb05e6db0ffc..2f27f8c5ff8e69679e507118ab14d0ad8794fe4e 100644
--- a/extensions/browser/api/networking_config/networking_config_service.cc
+++ b/extensions/browser/api/networking_config/networking_config_service.cc
@@ -25,7 +25,7 @@ bool IsValidNonEmptyHexString(const std::string& input) {
   if (count == 0 || (count % 2) != 0)
     return false;
   for (const char& c : input)
-    if (!IsHexDigit<char>(c))
+    if (!base::IsHexDigit<char>(c))
       return false;
   return true;
 }