[WebSocket] Create Peer on the worker thread.

This CL moves the WorkerThreadableWebSocketChannel::Peer construction to
the worker thread from the main thread.
That simplifies the construction and destruction of Peer and removes
#if ENABLE(OILPAN) macros.

BUG=384238

Review URL: https://codereview.chromium.org/350763007

git-svn-id: svn://svn.chromium.org/blink/trunk@178922 bbb929c8-8fbe-4397-9dbb-9b2b20218538