Bug 58952: Help incremental GC further by breaking refs

* convert some more document handlers to once()

* reset the HTML5 treebuilder with a new document immediately, but don't
  clobber the old document. TODO: Make it cleaner to reset the parser with a
  new document, for example by accepting a new document in parser.setup().

* Use getPipeline instead of makePipeline

* Use defaultOptions in getPipeline too

* Some cleanup around pipeline caching

* Adjust a parser test where the about ids have changed for some reason. TODO:
  investigate why those changed with pipeline caching.

Change-Id: Ie5db77fc701ec26b00a92600b5c0282ab02a15ad
Merge "Bug 58952: Help incremental GC further by breaking refs"