Implements a media.mozGetMetadata() method returning a new javascript object whose properties are key value pairs respresenting metadata tags from the media resource. This data is available after readystate enters METADATA_LOADED.
Currently this is only implemented for Ogg Vorbis streams.
Media format metadata is parsed out by the media decoders. In the nsCodecStateMachine::ReadMetadata subclasses we fill in an nsDataHashtable pointer using the format-specifc api.
The hash pointer is passed up to the media element as part of the MetadataLoaded event.
The hash is deleted if the load is aborted. The audio metadata is also reset to zero (as in the constructor), resolving a todo comment.
