Add multiarch specification to apt repo config.

Add multiarch specification to apt repo config.

TBR=phajdan.jr@chromium.org, thestig@chromium.org
BUG=591480
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1759243002

Cr-Commit-Position: refs/heads/master@{#379305}
(cherry picked from commit 9f2c239a6ece2f510fac2f1be1ca1811f0deb9a4)

Review URL: https://codereview.chromium.org/1769533003

Cr-Commit-Position: refs/branch-heads/2623@{#578}
Cr-Branched-From: 92d77538a86529ca35f9220bd3cd512cbea1f086-refs/heads/master@{#369907}
(cherry picked from commit 101ada9dd43913419b598e24f2044be2856946c3)

[Michael Moss, 2016-03-07 15:36:58]

The CQ bit was checked by mmoss@chromium.org

[commit-bot: I haz the power, 2016-03-07 15:37:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1766423002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1766423002/1

[commit-bot: I haz the power, 2016-03-07 15:38:47]

Committed patchset #1 (id:1)

[Lei Zhang, 2016-03-09 06:19:00]

rs lgtm