Require multiarch setting in Linux repo config file.

This will force old configs to add this setting on the next update, and
thus get rid of the "missing i386" warning for existing users.

R=phajdan.jr@chromium.org, thestig@chromium.org
BUG=591480

Review URL: https://codereview.chromium.org/1801283002

Cr-Commit-Position: refs/heads/master@{#381287}
(cherry picked from commit 8b58c0d445772371cba4da0000f78917a12c79bc)

Review URL: https://codereview.chromium.org/1804263002 .

Cr-Commit-Position: refs/branch-heads/2661@{#249}
Cr-Branched-From: ef6f6ae5e4c96622286b563658d5cd62a6cf1197-refs/heads/master@{#378081}
1 file changed