Oilpan: replace RawPtr field by Persistent.

The missing trace can cause use-after-free.

R=ager@chromium.org
TBR=ager@chromium.org, haraken@chromium.org
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/228413002

git-svn-id: svn://svn.chromium.org/blink/trunk@171048 bbb929c8-8fbe-4397-9dbb-9b2b20218538