Oilpan: fix WebSockets unit test GC plugin failure.

The MockWebSocketChannelClient's trace() method must delegate to the
mixin's trace() method (even if it is empty.)

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

Review URL: https://codereview.chromium.org/418803003

git-svn-id: svn://svn.chromium.org/blink/trunk@178826 bbb929c8-8fbe-4397-9dbb-9b2b20218538