Revert "Oilpan: Support the HTML parser thread in oilpan"

Speculative rollout of r170564 to see if unit_tests and content_unittests start working again.

TBR=haraken
BUG=340522

Review URL: https://codereview.chromium.org/219243014

git-svn-id: svn://svn.chromium.org/blink/trunk@170602 bbb929c8-8fbe-4397-9dbb-9b2b20218538