Oilpan: Test/example showing how to get the GC to remove empty sets from a map

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/327323008

git-svn-id: svn://svn.chromium.org/blink/trunk@176026 bbb929c8-8fbe-4397-9dbb-9b2b20218538