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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

Change in reland:
- Removes TCPSocketTest.ReadAndWriteMultiple because
  base::RunLoop().RunUntilIdle(), when used in conjunction with the rest of the
  test logic, times out sometimes on iOS bot. We have a better
  version of this test (ReadAndWriteMultipleWithMockClientSocket) written using
  mock sockets. See crbug.com/822058

R=mmenke@chromium.org
TBR=miu@chromium.org
TBR=tsepez@chromium.org

Bug: 721401, 822058
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I71cde7f2a16d022e888d62a6fec3d4fd5917b6e9
Reviewed-on: https://chromium-review.googlesource.com/964250
Commit-Queue: Helen Li <xunjieli@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543381}