Make linux_chromium_clobber_rel_ng experimental

Make linux_chromium_clobber_rel_ng experimental

This is needed to avoid tree breakages before making it mirror Linux GN Clobber.
Since currently, linux_chromium_clobber_rel_ng is mirroring a bot on FYI
waterfal it is not useful in CQ anyway.

R=phajdan.jr@chromium.org
BUG=531296
Committed: https://crrev.com/7ff7bb3be8f97dbcb1ed7e225fb7acdef15c3cc1
Cr-Commit-Position: refs/heads/master@{#354241}

[Paweł Hajdan Jr., 2015-10-15 08:16:44]

LGTM

[Sergiy Byelozyorov, 2015-10-15 08:17:03]

The CQ bit was checked by sergiyb@chromium.org

[commit-bot: I haz the power, 2015-10-15 08:17:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1403443004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1403443004/1

[commit-bot: I haz the power, 2015-10-15 09:30:11]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-15 09:30:47]

Patchset 1 (id:??) landed as
https://crrev.com/7ff7bb3be8f97dbcb1ed7e225fb7acdef15c3cc1
Cr-Commit-Position: refs/heads/master@{#354241}

[Sergiy Byelozyorov, 2015-10-20 12:02:16]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1420503003/ by sergiyb@chromium.org.

The reason for reverting is: Having a bot that mirrors clobber bot on FYI
waterfall is better than having nothing..