Fix field type in Cue.
Fix field type in Cue.

This field is typed to be a non-nullable string, but the compiler was
ignoring this type since it was defined on the interface.  A new
version of the compiler caught this error.  Originally cr/257309718.

Change-Id: I9e7589c3b059ca39324078c867dbc27f3742a095

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