Fix //tools/gn/example on Linux: missing current_os and related vars

Fix //tools/gn/example on Linux: missing current_os and related vars

The example GN project's BUILDCONIG.gn needs to set {target,current}_{os,cpu)
variables like the Chromium BUILDCONFIG file does. Otherwise some conditionals
don't work like they do in Chromium and the project doesn't link on Linux,
which is needlessly confusing.

To test, `gn gen out' in //tools/gn/example, and then `ninja -C out'.

R=brettw@chromium.org
BUG=

Committed: https://crrev.com/763cdd3e2a8143c6c4a7b0991579dd21aaf1aeac
Cr-Commit-Position: refs/heads/master@{#357329}

[brettw, 2015-11-02 08:37:29]

lgtm

[tsniatowski, 2015-11-02 09:55:51]

The CQ bit was checked by tsniatowski@opera.com

[commit-bot: I haz the power, 2015-11-02 09:56:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1429073002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1429073002/1

[commit-bot: I haz the power, 2015-11-02 10:50:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-02 10:51:01]

Patchset 1 (id:??) landed as
https://crrev.com/763cdd3e2a8143c6c4a7b0991579dd21aaf1aeac
Cr-Commit-Position: refs/heads/master@{#357329}