Merge pull request #2 from cablelabs/ruih/merge-kind-tag

Add the HTML5 track kind as GST_TAG_TRACK_KIND.
Revert "Add the HTML5 track kind as GST_TAG_TRACK_KIND."

This reverts commit d29576df1cc9492680b475886934a58e75b7cc2c.
Add GST_TAG_TRACK_KIND as a string.