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DNS histogram cleanup, part 3

This CL removes all the old DNS.Resolve* and AsyncDNS.Resolve*
histograms and replaces them with a new set. The new Net.DNS.Resolve*
histograms measure timing and success/failure at the Job layer, and each
type of Task gets its own set to measure Task layer timing and
success/failure.

Bug: 769320
Change-Id: I0b0df897310a474fd11022f932724c8d40789090
Reviewed-on: https://chromium-review.googlesource.com/687836
Commit-Queue: Miriam Gershenson <mgersh@chromium.org>
Reviewed-by: Julia Tuttle <juliatuttle@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505215}