Oilpan: Register Oilpan heap pages as LSan root pages.

This change fixes LSan false alarms for Oilpan-related objects.

BUG=378404

Review URL: https://codereview.chromium.org/314533002

git-svn-id: svn://svn.chromium.org/blink/trunk@175334 bbb929c8-8fbe-4397-9dbb-9b2b20218538