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This CL merges the higher-level DNS histograms to include both async and
system resolver timing in the same metrics. This will allow easier use
in Finch experiments. It also adds the new Net.DNS.TotalTimeNotCached
histogram, which is equivalent to Net.DNS.TotalTime minus cached and
other synchronous resolutions, for better alerts. The current TotalTime
histogram is hard to use for alerts because the median is 0.

Bug: 769320
Change-Id: Iea7873ddd284fffe1e0e414f1e33f2e07053c0ca
Reviewed-on: https://chromium-review.googlesource.com/673092
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@{#505119}