test: Fix broken document.createElement calls in log-window-tests.js
test: Fix broken document.createElement calls in log-window-tests.js

I broke these tests in a refactor in 86c72c06, where I changed which
"document" a log window referred to.  This does not appear to have had
any effect on a production environment.

Change-Id: Icd4fedf9239192aa979d22a721fc6965347a101f

shaka-project/eme_logger,shaka-project/eme_logger