Restores SampleDataEvent to create a ByteArray.

From [Adobe's example](http://help.adobe.com/en_US/as3/dev/WSE523B839-C626-4983-B9C0-07CF1A087ED7.html), SampleDataEvents already contain an allocated ByteArray.

[Previous versions of openfl](https://github.com/openfl/openfl-native/blob/master/flash/events/SampleDataEvent.hx) also work this way. I don't know where the source history is that contains commit that commented these lines. In this repo, they've always been commented out. I don't know why it was changed.

This restores the functionality to match the flash API.