The HTML parser should ignore incoming bytes when stopped, not just when detached, otherwise we might redundantly re-start the background parser.

Also added a few asserts to catch any future causes of the linked crash.

BUG=329603
R=abarth@chromium.org,eseidel@chromium.org

Review URL: https://codereview.chromium.org/133173003

git-svn-id: svn://svn.chromium.org/blink/trunk@165564 bbb929c8-8fbe-4397-9dbb-9b2b20218538