Add HTMLConstructionSite::queueTask and processEndOfFile methods

This is just a small part of:
https://codereview.chromium.org/26129005/
which I'm having trouble landing in one go w/o causing strange regressions.

R=abarth@chromium.org
BUG=301981

Review URL: https://codereview.chromium.org/26549003

git-svn-id: svn://svn.chromium.org/blink/trunk@159378 bbb929c8-8fbe-4397-9dbb-9b2b20218538