Oilpan: fix reported unit test leaks.

Make some unit tests not rely on any external code triggering a GC to
tidy up the resources they allocate on the heap. Prevents reporting of
leaked test objects.

R=haraken@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/412173002

git-svn-id: svn://svn.chromium.org/blink/trunk@178830 bbb929c8-8fbe-4397-9dbb-9b2b20218538