Oilpan: Ignore AsyncMethodRunner::m_object in Oilpan's plugin

Oilpan's pulgin complains that AsyncMethodRunner::m_object should be traced
probably due to a bug of the plugin. This CL suppresses the complaint.

BUG=363031

Review URL: https://codereview.chromium.org/235703006

git-svn-id: svn://svn.chromium.org/blink/trunk@171515 bbb929c8-8fbe-4397-9dbb-9b2b20218538