Oilpan: Add Source/platform/heap/ to WATCHLISTS

TBR=ager@chromium.org

Review URL: https://codereview.chromium.org/232683002

git-svn-id: svn://svn.chromium.org/blink/trunk@171242 bbb929c8-8fbe-4397-9dbb-9b2b20218538