Add per-file wildcard for chromeos/BUILD.gn

Add per-file wildcard for chromeos/BUILD.gn

So that changes to this file do not need approval from
chromeos/OWNERS

BUG=none

Committed: https://crrev.com/58abfe647e91f920e304f79f8959c8f9bd01d552
Cr-Commit-Position: refs/heads/master@{#313598}

[satorux1, 2015-01-28 05:04:53]

satorux@chromium.org changed reviewers:
+ oshima@chromium.org

[oshima, 2015-01-28 17:57:57]

lgtm

[satorux1, 2015-01-28 22:18:15]

The CQ bit was checked by satorux@chromium.org

[commit-bot: I haz the power, 2015-01-28 22:19:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/882903003/1

[commit-bot: I haz the power, 2015-01-28 22:51:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-28 22:52:37]

Patchset 1 (id:??) landed as
https://crrev.com/58abfe647e91f920e304f79f8959c8f9bd01d552
Cr-Commit-Position: refs/heads/master@{#313598}