Decouple WebSocketChannelClient from WebSocketImpl

Ideally we want to simply make WebSocketImpl inherit from WebSocketChannelClient,
but we cannot do that because WebSocketChannelClient needs to be on Oilpan's heap
whereas WebSocketImpl cannot be on Oilpan's heap. Thus this CL introduces a proxy
class to decouple WebSocketImpl from WebSocketChannelClient.

BUG=340522

Review URL: https://codereview.chromium.org/419973007

git-svn-id: svn://svn.chromium.org/blink/trunk@179460 bbb929c8-8fbe-4397-9dbb-9b2b20218538