Oilpan: Move ThreadableWebSocketChannelClientWrapper to Oilpan's heap

This is a preparation for moving WebSocketChannelClient to the heap.

This CL adds a template specialization to CrossThreadCopier.h so that ThreadableWebSocketChannelClientWrapper* can be passed to createCallbackTask().

BUG=340522

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174956

Review URL: https://codereview.chromium.org/267323004

git-svn-id: svn://svn.chromium.org/blink/trunk@175782 bbb929c8-8fbe-4397-9dbb-9b2b20218538