dhcpcd: Request and report Web Proxy Auto-Discovery

Request the Web Proxy Auto-Discovery DHCP option, and report
any retrieved value over over DBus.

BUG=chromium:253915
TEST=Packet capture from network_DhcpNegotiationSuccess autotest
Turn on dhcp debugging in shill and note that a WebProxyAutoDiscoveryUrl
key is received (but ignored).

Change-Id: Ib3a0fe4fd4559c7f61545e2a64077265adf0fe7c
Reviewed-on: https://gerrit.chromium.org/gerrit/59967
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
3 files changed