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 8b58c0d)

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

Cr-Commit-Position: refs/branch-heads/2661@{crosswalk-project#249}
Cr-Branched-From: ef6f6ae-refs/heads/master@{#378081}

Author: mmossg

chrome/installer/linux/debian/build.sh

@@ -339,7 +339,7 @@ REPOCONFIG="${REPOCONFIG-deb [arch=${ARCHITECTURE}] http://${BASEREPOCONFIG}}"
 # Allowed configs include optional HTTPS support and explicit multiarch
 # platforms.
 REPOCONFIGREGEX="deb (\\\\[arch=[^]]*\\\\b${ARCHITECTURE}\\\\b[^]]*\\\\]"
-REPOCONFIGREGEX+="[[:space:]]*)?https?://${BASEREPOCONFIG}"
+REPOCONFIGREGEX+="[[:space:]]*) https?://${BASEREPOCONFIG}"
 stage_install_debian
 
 do_package