Fixed a bug of GStreamer.
Fixed a bug of GStreamer.

* GStreamer is a subclass of CVFile, but it did not have self._source_file.
* It raised an error on the get_start_time function.

deathmetalland/IkaLog,hasegaw/IkaLog,deathmetalland/IkaLog,deathmetalland/IkaLog,hasegaw/IkaLog,hasegaw/IkaLog