Add compile-time flag for oilpan.

First patch to prepare for landing the Oilpan infrastructure files.

R=abarth@chromium.org, darin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/103613003

git-svn-id: svn://svn.chromium.org/blink/trunk@163403 bbb929c8-8fbe-4397-9dbb-9b2b20218538