TextDisplayer: enable showing captions in multiple lines
TextDisplayer: enable showing captions in multiple lines

Set css style white-space to 'pre', so that when the captions text
contains a line break '\n', the captions will display in seperate lines.

Change-Id: I623d48ab2bbd82bb9dd3f2565c723d6ce35285e1

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