This is based on network driver populated skb->mark magic bit.
This is the bit used by netd's WakeupController.

We mandated the location of this bit in U, though we haven't
(yet??) mandated it being supported by all network drivers.

If the driver doesn't support it, it could always
be false (skb->mark should default to 0),
or potentially (this is very very unlikely) be garbage.

IFIRC nettrace isn't enabled on pre-U devices anyway.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I2b3b91315d77c08c022396253b26954593dd3f5a