Remove non-cue elements from text displayer.
Remove non-cue elements from text displayer.

Sometimes we see extra DOM elements in the text container.  These may
appear for some reason when seeking (may not be removed completely).
This works around that by removing any existing DOM elements that don't
belong to an existing cue.

Issue #2076

Change-Id: Iab0210cf5d046a818d1e23fc02947f3550622843

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