Implement isAudioOnly for src=
Implement isAudioOnly for src=

This is a generic implementation that doesn't rely on the audioTracks
and videoTracks members currently only available on Safari.

Issue #997

Change-Id: I849845513efb2ee51205dcdca8568c889f1f7cdb

tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player