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As a preparation to integrate websocket with the network service, this
CL moves the mojom file to /services/network.

Bug: 721400
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I1284cbd9dd3292c41d1acba34835c6c6d44d48a3
Reviewed-on: https://chromium-review.googlesource.com/961173Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543632}