Sometimes fails to detect video elements

[joeyparrish]

In some cases, the code to detect video elements seems to fail. It should work like this:

1. Query the document for all existing video elements (synchronous, in _traceExistingElements)
2. Listen for DOM mutations to catch future video elements (in _setupNewElementObserver)

Somehow, there is a hole in this that allows some video elements to escape notice some of the time. On some pages, reloading a few times will eventually make it work again.