Add a mode to OAuth2MintTokenFlow that fetches the messages to show to the user.

chrome/common/net/gaia/oauth2_access_token_fetcher.cc

Index: chrome/common/net/gaia/oauth2_access_token_fetcher.cc
===================================================================
--- chrome/common/net/gaia/oauth2_access_token_fetcher.cc	(revision 130985)
+++ chrome/common/net/gaia/oauth2_access_token_fetcher.cc	(working copy)
@@ -42,20 +42,6 @@
 
 static const char kAccessTokenKey[] = "access_token";
 
-static bool GetStringFromDictionary(const DictionaryValue* dict,
-                                    const std::string& key,
-                                    std::string* value) {
-  Value* json_value;
-  if (!dict->Get(key, &json_value))
-    return false;
-  if (json_value->GetType() != base::Value::TYPE_STRING)
-    return false;
-
-  StringValue* json_str_value = static_cast<StringValue*>(json_value);
-  json_str_value->GetAsString(value);
-  return true;
-}
-
 static GoogleServiceAuthError CreateAuthError(URLRequestStatus status) {
   CHECK(!status.is_success());
   if (status.status() == URLRequestStatus::CANCELED) {
@@ -211,5 +197,5 @@
     return false;
 
   DictionaryValue* dict = static_cast<DictionaryValue*>(value.get());
-  return GetStringFromDictionary(dict, kAccessTokenKey, access_token);
+  return dict->GetString(kAccessTokenKey, access_token);
 }