Oilpan: fix WebSocket unit tests memory leaks.

Remove unnecessary Persistent reference (to an object contained within
another Persistent) + arrange for manual GCing of resources to avoid
test objects being reported as not destructed and leaking with
Oilpan enabled.

R=
BUG=

Review URL: https://codereview.chromium.org/306273002

git-svn-id: svn://svn.chromium.org/blink/trunk@175233 bbb929c8-8fbe-4397-9dbb-9b2b20218538