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

I broke these tests in a refactor in 86c72c0, 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

spec/log-window-tests.js

@@ -23,6 +23,7 @@ describe('Log window', () => {
 
   beforeAll(() => {
     mockDocument = document.createElement('div');
+    mockDocument.createElement = (name) => document.createElement(name);
     document.body.appendChild(mockDocument);
 
     mockWindow = {