Add LeakExpectations

This is for the leak detector bots which are being implemented at
https://codereview.chromium.org/224963004/.

BUG=359024

Review URL: https://codereview.chromium.org/227463002

git-svn-id: svn://svn.chromium.org/blink/trunk@171011 bbb929c8-8fbe-4397-9dbb-9b2b20218538