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Added histograms "Network.Shill.Cellular.IPv6ConnectivityStatus" and
"Network.Shill.Wifi.IPv6ConnectivityStatus" for tracking the IPv6
connectivity status when network connection is established. This
allows us to know the readiness of IPv6 connectivity in the field.

Added histograms "Network.Shill.Cellular.NetworkConnectionIPType" and
"Network.Shill.Wifi.NetworkConnectionIPType" for tracking the types of
IP configuration use for establishing network connections. This allows
us to track the usage of the newly added IPv6 connection support.

BUG=chromium:207192

Review URL: https://codereview.chromium.org/539163003

Cr-Commit-Position: refs/heads/master@{#293372}