Linux installer: Fix APT_SOURCES and APT_SOURCESDIR paths

In prior versions of Ubuntu/Debian the Dir::Etc apt-config variable contained a trailing slash by default. The cronjob at /etc/cron.daily/google-chrome was concatenating that variable into a path, assuming the trailing slash. But in Ubuntu 16.10, the trailing slash is gone. That breaks the path to the list file, and the cronjob fails to fix it.

BUG=663069

Review-Url: https://codereview.chromium.org/2482343002
Cr-Commit-Position: refs/heads/master@{#430771}