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|dns_resolution_end_time_| is still used in other histograms
(e.g. Net.QuicServerInfo.DiskCacheWaitForDataReadyTime and
Net.QuicSession.HostResolution.HandshakeConfirmedTime).

The initialization is removed in crrev.com/2256803002. This CL adds it back
to maintain correctness of the remaining histograms which are still using this
field.

R=zhongyi@chromium.org

BUG=629384, 637051

Review-Url: https://codereview.chromium.org/2314403006
Cr-Commit-Position: refs/heads/master@{#417355}