Return track.src as a url.

This normalizes whatever is set as track.src to a full uri,
so base is applies, relative paths resolved, etc. Review
request from bz. Opened bug 872271 to add a test for this.
Hoist NewURIFromString entirely to nsGenericHTMLElement.

Previously HTMLMediaElement and nsGenericHTMLElement had
separate copies of this helper. Make HTMLMediaElement use
the same copy as HTMLTrackElement.

Review request from bz. Please double-check the NS_RELEASE
addition in the promoted version.