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It had a single implementation, FCMSyncNetworkChannel, and nothing
actually referred to the interface.

Bug: 1029481
Change-Id: I4791dd4a4760687f29499faee7986d1dfefb5745
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062422
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743029}