Add OilpanLeakExpectation

This is for the Oilpan leak detector bot which is being implemented at
https://codereview.chromium.org/250533002/

BUG=359024
NOTRY=true

Review URL: https://codereview.chromium.org/255573004

git-svn-id: svn://svn.chromium.org/blink/trunk@172581 bbb929c8-8fbe-4397-9dbb-9b2b20218538