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Was just used for MappedHostResolver, but moving to HostResolver to be
more generally available to HostResolver implementations.

Also rename to FailingRequestImpl.

Bug: 1006902
Change-Id: Ifffe9971057d8e93a653593d93a6dc88e053f8f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1828099
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701250}