Add explicit base namespace to string16 users.

chrome/common/net/x509_certificate_model.cc

Index: chrome/common/net/x509_certificate_model.cc
diff --git a/chrome/common/net/x509_certificate_model.cc b/chrome/common/net/x509_certificate_model.cc
index 416bd9dcc99085fc71594df3d960eaeb7f141b53..bfc119217beb4baf99f1890772fd0de6e33adad5 100644
--- a/chrome/common/net/x509_certificate_model.cc
+++ b/chrome/common/net/x509_certificate_model.cc
@@ -17,11 +17,11 @@ namespace x509_certificate_model {
 
 std::string ProcessIDN(const std::string& input) {
   // Convert the ASCII input to a string16 for ICU.
-  string16 input16;
+  base::string16 input16;
   input16.reserve(input.length());
   input16.insert(input16.end(), input.begin(), input.end());
 
-  string16 output16 = net::IDNToUnicode(input, std::string());
+  base::string16 output16 = net::IDNToUnicode(input, std::string());
   if (input16 == output16)
     return input;  // Input did not contain any encoded data.