fix(period flattening): Combine text streams with no roles
fix(period flattening): Combine text streams with no roles

If two text streams in two periods have no roles while other text
streams have, combine those with no roles.

Issue #2785
b/172605645

Change-Id: Ie3b98e7941d38f5be90f4eb65d1a3de1c3e394a3

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