Use std::string for decryption key ID in webm parser.

media/webm/webm_cluster_parser_unittest.cc

Index: media/webm/webm_cluster_parser_unittest.cc
diff --git a/media/webm/webm_cluster_parser_unittest.cc b/media/webm/webm_cluster_parser_unittest.cc
index 7ed6c6c238e535a9c948973c4859561cfe766ac3..3737e38f255e9817e86faa43174d4d1bec0fdfac 100644
--- a/media/webm/webm_cluster_parser_unittest.cc
+++ b/media/webm/webm_cluster_parser_unittest.cc
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <algorithm>
+
 #include "base/logging.h"
 #include "media/webm/cluster_builder.h"
 #include "media/webm/webm_cluster_parser.h"
@@ -125,7 +127,7 @@ class WebMClusterParserTest  : public testing::Test {
       : parser_(new WebMClusterParser(kTimecodeScale,
                                       kAudioTrackNum,
                                       kVideoTrackNum,
-                                      NULL, 0)) {
+                                      std::string())) {
   }
 
  protected: