Peer::m_syncHelper should be cleared when the Peer is destroyed

This is a speculative fix for crashes in websocket tests in oilpan bots.

Peer::m_syncHelper should be cleared when the Peer object is destroyed. Otherwise, the main thread keeps retaining a Member to the on-heap SyncHelper object allocated in the worker thread's heap even after the worker thread shuts down. This is not allowed in oilpan. All references to on-heap objects on worker thread's heap should be cleared before the worker thread shuts down.

BUG=396503

Review URL: https://codereview.chromium.org/411543008

git-svn-id: svn://svn.chromium.org/blink/trunk@178789 bbb929c8-8fbe-4397-9dbb-9b2b20218538