Add timeline ad UI tests.
Add timeline ad UI tests.

Issue #2367.

Change-Id: I49042992f4462f35e48292e79a09fef779494faa

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