Enable turning off captions
Enable turning off captions

Closes #1893

Change-Id: I65864e4184b0129351bbf7c3776f9019ccc09f6b

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