Revert of Enforce "set noparent" on metrics XML files.

Revert of Enforce "set noparent" on metrics XML files. (patchset #1 id:20001 of https://codereview.chromium.org/1443463002/ )

Reason for revert:
I think this broke owners checks for metrics xml files.

Original issue's description:
> Enforce "set noparent" on metrics XML files.
>
> This restores this enforcement that was recently removed from
> tools/metrics by: https://codereview.chromium.org/1437683006/
>
> See discussion on that CL for more context.
>
> Also, removes jar@ from tools/metrics/OWNERS as he no longer
> actively does metrics reviews in Chromium.
>
> BUG=none
>
> Committed: https://crrev.com/b2ced0815d5e9e2d3265ff116d45719261906c4e
> Cr-Commit-Position: refs/heads/master@{#359625}

TBR=isherman@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=none

Committed: https://crrev.com/e73a707fd17358a98f8c972d75314408bcbd23d6
Cr-Commit-Position: refs/heads/master@{#359646}

[Alexei Svitkine (slow), 2015-11-13 21:23:52]

Created Revert of Enforce "set noparent" on metrics XML files.

[commit-bot: I haz the power, 2015-11-13 21:24:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1448523002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1448523002/1

[Alexei Svitkine (slow), 2015-11-13 21:24:35]

Suspecting this is causing this stack trace when doing "git cl upload" with a
histograms.xml change:

Running presubmit upload checks ...
Traceback (most recent call last):
  File "/Users/asvitkine/depot_tools/git_cl.py", line 3676, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/Users/asvitkine/depot_tools/git_cl.py", line 3658, in main
    return dispatcher.execute(OptionParser(), argv)
  File "/Users/asvitkine/depot_tools/subcommand.py", line 252, in execute
    return command(parser, args[1:])
  File "/Users/asvitkine/depot_tools/git_cl.py", line 2418, in CMDupload
    change=change)
  File "/Users/asvitkine/depot_tools/git_cl.py", line 1109, in RunHook
    rietveld_obj=self.RpcServer())
  File "/Users/asvitkine/depot_tools/presubmit_support.py", line 1477, in
DoPresubmitChecks
    results += executer.ExecPresubmitScript(presubmit_script, filename)
  File "/Users/asvitkine/depot_tools/presubmit_support.py", line 1394, in
ExecPresubmitScript
    result = eval(function_name + '(*__args)', context)
  File "<string>", line 1, in <module>
  File "<string>", line 1916, in CheckChangeOnUpload
  File "<string>", line 1653, in _CommonChecks
  File "/Users/asvitkine/depot_tools/presubmit_canned_checks.py", line 1092, in
PanProjectChecks
    input_api, output_api, source_file_filter=None))
  File "/Users/asvitkine/depot_tools/presubmit_canned_checks.py", line 926, in
CheckOwners
    suggested_owners = owners_db.reviewers_for(missing_files, owner_email)
  File "/Users/asvitkine/depot_tools/owners.py", line 138, in reviewers_for
    suggested_owners = self._covering_set_of_owners_for(files, author)
  File "/Users/asvitkine/depot_tools/owners.py", line 309, in
_covering_set_of_owners_for
    owner = self.lowest_cost_owner(all_possible_owners, dirs_remaining)
  File "/Users/asvitkine/depot_tools/owners.py", line 363, in lowest_cost_owner
    lowest_cost = min(total_costs_by_owner.itervalues())
ValueError: min() arg is an empty sequence

[commit-bot: I haz the power, 2015-11-13 21:27:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-13 21:27:45]

Patchset 1 (id:??) landed as
https://crrev.com/e73a707fd17358a98f8c972d75314408bcbd23d6
Cr-Commit-Position: refs/heads/master@{#359646}