Oilpan: fix WebSockets unit tests after r178758.

Explicitly disconnect the channel on test completion, putting the heap
object in the expected state by the time the next GC sweep happens.

TBR=oilpan-reviews@chromium.org,tyoshino@chromium.org,yhirano@chromium.org,hiroshige@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/410263002

git-svn-id: svn://svn.chromium.org/blink/trunk@178778 bbb929c8-8fbe-4397-9dbb-9b2b20218538