Add default css styles for captions
Add default css styles for captions

Issue #1708

Change-Id: Ie3fec157c789b4626ee098d6794a8ca75dc561e0

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