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This CL adds TCP socket API to network service.
This is partly based on mandoline implementation
(https://codereview.chromium.org/1873463003/)

Two TCP socket interfaces are added.
- TCPConnectedSocket
- TCPServerSocket

TBR=miu@chromium.org

Bug: 721401
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I56e2702c02b9c86a72eba68c85391c3b017a4d64
Reviewed-on: https://chromium-review.googlesource.com/868711Reviewed-by: Helen Li <xunjieli@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Commit-Queue: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542606}