Oilpan: Hide callback-stack blocks behind an abstraction and add constant-time append.

R=ager@chromium.org, erik.corry@gmail.com, wibling@chromium.org
BUG=

Review URL: https://codereview.chromium.org/594543003

git-svn-id: svn://svn.chromium.org/blink/trunk@182499 bbb929c8-8fbe-4397-9dbb-9b2b20218538