Stop the background parser when parsing is canceled to avoid ASSERTing in ~HTMLDocumentParser().
Make the threaded parser slightly faster by letting it yield every 4000 tokens.  I didn't tune that number very much.  Only enough to find that every 100 tokens and every 1000000 tokens were slower than 1000 or 4000.  I picked 4000 based on the fact that the single-threaded parser checks for its 500ms yield-timeout every 4000 tokens.
Add a version of the html-parser benchmark which uses srcdoc instead of document.write so that we can actually test the threaded html parser.