Oilpan: Perform weak processing of full objects on the owning thread.

This is needed because the general weak processing can lead to the
deallocation of objects and therefore has to happen on the owning
thread.

R=erik.corry@gmail.com, oilpan-reviews@chromium.org, vegorov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/188673002

git-svn-id: svn://svn.chromium.org/blink/trunk@168834 bbb929c8-8fbe-4397-9dbb-9b2b20218538