cros_portage_upgrade: stop blacklisting metadata.xml
cros_portage_upgrade: stop blacklisting metadata.xml

We want to start carrying this file as it contains information the
security team wants to track (CPE URIs).  Let cros_portage_upgrade
pull it in & update it for us.

BUG=None
TEST=None

Change-Id: I1195aaf37f5dbc68228eb1cd12b62fad9cd60020
Reviewed-on: https://gerrit.chromium.org/gerrit/50679
Reviewed-by: Jim Hebert <ba29f3d391c58b853c47a49b4da91f3ec0815c1c@chromium.org>
Reviewed-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite