Only run install-debian.wheezy.sysroot.py once during runhooks

Only run install-debian.wheezy.sysroot.py once during runhooks

This script was previously run three times, once for
each arch, with two of the three always being no-ops.

Instead just run the script once and have the script
figure out which image to download.

TEST=trybots + 'gclient runhooks' locally

Committed: https://crrev.com/711802679ce2feb228ad418dd117ae23301e5c02
Cr-Commit-Position: refs/heads/master@{#310467}

[Sam Clegg, 2015-01-07 14:03:07]

sbc@chromium.org changed reviewers:
+ thestig@chromium.org

[Lei Zhang, 2015-01-07 22:45:48]

lgtm

Keep an eye on the "official" bots and make sure they don't blow up.

[Sam Clegg, 2015-01-08 10:11:40]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-01-08 10:12:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/834133003/20001

[commit-bot: I haz the power, 2015-01-08 10:23:27]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-08 10:24:10]

Patchset 2 (id:??) landed as
https://crrev.com/711802679ce2feb228ad418dd117ae23301e5c02
Cr-Commit-Position: refs/heads/master@{#310467}

[henrika (OOO until Aug 14), 2015-01-08 10:46:14]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/843683003/ by henrika@chromium.org.

The reason for reverting is: See failing gclient runhooks after this patch and
the Chrome build tree turned red (see
http://build.chromium.org/p/chromium.win/waterfall?builder=Win8%20GN for
details).

I can't find any other reason for this failure and will will try to revert..