Oilpan: make DocumentFragment a heap allocated object.

This relands r174382, as it was reverted while investigating a test failure.

R=wibling@chromium.org,zerny@chromium.org,haraken@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/289273002

git-svn-id: svn://svn.chromium.org/blink/trunk@174443 bbb929c8-8fbe-4397-9dbb-9b2b20218538