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Follow up to https://crrev.com/c/2289191.

This callback is only used in tether code as an error callback. This
change moves the callback to TetherDelegate and renames to
StringErrorCallback. Even though this is mostly used in
chromeos/components/tether, it can't be defined there since
chromeos/network can't include code from there.

Bug: 1007660
Change-Id: I882d25857afdcc2b86a9cae4f146032603b6d40f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303871Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789263}