download_gold_plugin.py: Redirect stderr to /dev/null when calling gsutil.

download_gold_plugin.py: Redirect stderr to /dev/null when calling gsutil.

When called from a bot, runhooks' stdin will be redirected to /dev/null. The
hack we use in update.py that redirects stdin to stderr causes a read-only
/dev/null to eventually appear as gsutil's stderr here, which causes it
to blow up for an unknown reason. To avoid this problem, open a writable
/dev/null and redirect unconditionally here.

This prevents command line users from seeing gsutil's output, but at least
the thing runs.

BUG=453195
R=thakis@chromium.org

Committed: https://crrev.com/83a9be53371d0bc5a1084e2162cb28b2d073433e
Cr-Commit-Position: refs/heads/master@{#339542}

[Nico, 2015-07-20 20:00:47]

lgtm

Maybe you can add some "remove this once crbug.com/494442 is fixed" type of
comment (494442 should allow us to get rid of that stdin->stderr hack)

[pcc1, 2015-07-20 22:24:13]

The CQ bit was checked by pcc@chromium.org

[pcc1, 2015-07-20 22:24:13]

The patchset sent to the CQ was uploaded after l-g-t-m from thakis@chromium.org
Link to the patchset: https://codereview.chromium.org/1246783002/#ps20001
(title: "Update comment")

[commit-bot: I haz the power, 2015-07-20 22:25:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1246783002/20001

[commit-bot: I haz the power, 2015-07-20 22:39:24]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-07-20 22:40:54]

Patchset 2 (id:??) landed as
https://crrev.com/83a9be53371d0bc5a1084e2162cb28b2d073433e
Cr-Commit-Position: refs/heads/master@{#339542}