[oilpan] Null pointers must be treated as live, since they can't be marked live by strongification

R=ager@chromium.org, haraken@chromium.org
BUG=402426
NOTRY=true

Review URL: https://codereview.chromium.org/464283002

git-svn-id: svn://svn.chromium.org/blink/trunk@180173 bbb929c8-8fbe-4397-9dbb-9b2b20218538