This is similar to what the JS version does.
The TreeWalker and NodeFilter classes are adapted from
https://github.com/Krinkle/dom-TreeWalker-polyfill
(MIT license).
This makes #getComments twice as fast on en-big-oldparser.html
Change-Id: I2441f33e6e7bad753ac830d277e6a2e81ee8c93d