Revert 174382 "Oilpan: make DocumentFragment a heap allocated ob..."

Suspect this caused assertion failures in inspector/split-view.html,
which may indicate a deeper problem. See Issue 357163.

> Oilpan: make DocumentFragment a heap allocated object.
> 
> R=wibling@chromium.org,zerny@chromium.org,haraken@chromium.org
> BUG=357163
> 
> Review URL: https://codereview.chromium.org/289273002

TBR=sigbjornf@opera.com

Review URL: https://codereview.chromium.org/291233003

git-svn-id: svn://svn.chromium.org/blink/trunk@174396 bbb929c8-8fbe-4397-9dbb-9b2b20218538