Oilpan: Prepare to move HTMLResourcePreloader to Oilpan heap.

We can trace a Document member.

BUG=357163

Review URL: https://codereview.chromium.org/387863002

git-svn-id: svn://svn.chromium.org/blink/trunk@178014 bbb929c8-8fbe-4397-9dbb-9b2b20218538