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This patch adds a number of fields to NetworkConfiguration objects
defined in net.mojom in order to pass to ARC information it
currently lacks or infers from the environment:

 - Explictly passing the host IPv4 configuration and DNS addresses:
this effectively replaces the ip_configs field which is both
incovenient to parse on ARC side and not necessary for IPv6 outside of
DNS. Instead it is more straightforward and easier on both sides to
directly pass the IPv4 configuration and IPv4 + IPv6 DNS settings. This
matches better with the interface of android.net.LinkProperties.

- List of DNS search domains: similarly to DNS addresses, ARC should use
the settings set for the host during layer3 provisioning or set
statically. This is important for managed networks and necessary for
resolving "short" hostnames inside ARC. Like DNS addresses, search
domains are aggregated over both v4 and v6 and passed as an explicit
array.

- Static IPv4 configuration for ARC: this replaces the legacy mechanism
of arc-networkd setting the IPv4 address assigned to an Android virtual
network by directly entering the ARC container namespace and setting the
address on Android virtual interface. This is necessary for using ARC's
ArcNetworkService in ARCVM where ns-enter cannot be used.

- MTU value: ARC currently does not set the MTU correctly for network
 with non-standard MTUs (like some cellular networks) and always use a
 default MTU value.

Follow-up patches will start populating these fields.

BUG=b:123431422
BUG=b:143258259
BUG=b:145960788
BUG=crbug:795603
TEST=Compiled

Change-Id: I08dfd5daa9ba2946a847e555bb94a01da3866eb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1959203Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Hugo Benichi <hugobenichi@google.com>
Cr-Commit-Position: refs/heads/master@{#739163}