WebWorker control connections can transfer handles from the browser
process to the worker process, including RequestServer and ImageDecoder
transport handles sent during worker startup. On Windows, serializing
those attachments needs the destination process id so DuplicateHandle
and WSADuplicateSocketW can target the peer process. WebWorker was
excluded from the generic helper-process InitTransport handshake,
leaving the transport without a peer pid before any attachment-bearing
message was sent.
Add InitTransport to the WebWorker server endpoint, implement the
server-side peer pid exchange, expose the message type through
WebWorkerClient, and let the shared helper launcher perform the
handshake for workers as it does for other Windows IPC clients.
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