[WebSocket] Reset the WebPrivatePtr on destruction.

WebPrivatePtr and WebPrivateOwnPtr should be reset manually on destruction.

BUG=310405
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/114023003

git-svn-id: svn://svn.chromium.org/blink/trunk@163856 bbb929c8-8fbe-4397-9dbb-9b2b20218538