Log MediaSource parsing errors to the MediaLog so they can appear in chrome:media-internals.

media/base/media_log_event.h

Index: media/base/media_log_event.h
diff --git a/media/base/media_log_event.h b/media/base/media_log_event.h
index 4424b6849a8d858bd5dd948f97fe4b32df84e722..9b0f6e1d96137c8f71bcdfcf7febe824807f7f06 100644
--- a/media/base/media_log_event.h
+++ b/media/base/media_log_event.h
@@ -68,6 +68,10 @@ struct MediaLogEvent {
     //         "buffer_current": <current offset>.
     //         "buffer_end": <last buffered byte>.
     BUFFERED_EXTENTS_CHANGED,
+
+    // Errors reported by Media Source Extensions code.
+    MEDIA_SOURCE_ERROR,
+    // params: "error": Error string describing the error detected.
   };
 
   int32 id;