Right now VoidDnsWriter is injected in the tools - meaning it's possible to
*set* VoidDnsWriter as the writer of a TLD - but it isn't injected in the
backend - meaning we get an error if we actually try to use it.
We need VoidDnsWriter at least for load-testing, and in general for any test
TLD.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179425574