HttpNetworkTransaction handles asynchronous auth token generation.

BUG=21050,42222
TEST=net_unittests

Review URL: http://codereview.chromium.org/2722009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@50088 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/net/http/http_auth_handler.cc b/net/http/http_auth_handler.cc
index cf690bc2..fe6f91d 100644
--- a/net/http/http_auth_handler.cc
+++ b/net/http/http_auth_handler.cc
@@ -37,10 +37,11 @@
                                        const HttpRequestInfo* request,
                                        CompletionCallback* callback,
                                        std::string* auth_token) {
-  // TODO(cbentzel): Enforce non-NULL callback+auth_token.
+  // TODO(cbentzel): Enforce non-NULL callback after cleaning up SocketStream.
   DCHECK(request);
   DCHECK((username == NULL) == (password == NULL));
   DCHECK(username != NULL || AllowsDefaultCredentials());
+  DCHECK(auth_token != NULL);
   return GenerateAuthTokenImpl(username, password, request, callback,
                                auth_token);
 }