Repro for crbug.com/1182010: ExecuteScript intermittently Fails to Inject JavaScript for Xml-Based Documents

Repro steps:

1. Install the extension indicated in the bug
2. Refresh this page
3. Open DevTools and note per the log statements that the extension content script was injected < 4 times, but it should have been injected into all 3 iframes on the page (plus the top-level page). Note: occasionally it won't repro. In this case, refresh the page a few times; eventually you should observe the bug.