base: Get rid of 'using' declaration of StringAppendF.

net/proxy/proxy_bypass_rules.cc

Index: net/proxy/proxy_bypass_rules.cc
diff --git a/net/proxy/proxy_bypass_rules.cc b/net/proxy/proxy_bypass_rules.cc
index d80e6f177c8ab8198fc4910d6aad4e61f06b9c75..ce018bfcef903a5e120c6e3334b9e8d476641062 100644
--- a/net/proxy/proxy_bypass_rules.cc
+++ b/net/proxy/proxy_bypass_rules.cc
@@ -38,10 +38,10 @@ class HostnamePatternRule : public ProxyBypassRules::Rule {
   virtual std::string ToString() const {
     std::string str;
     if (!optional_scheme_.empty())
-      StringAppendF(&str, "%s://", optional_scheme_.c_str());
+      base::StringAppendF(&str, "%s://", optional_scheme_.c_str());
     str += hostname_pattern_;
     if (optional_port_ != -1)
-      StringAppendF(&str, ":%d", optional_port_);
+      base::StringAppendF(&str, ":%d", optional_port_);
     return str;
   }