Synchronize localtime between Chrome OS and host OS:
 - bindmount /var/lib/timezone
 - During installation, link /etc/localtime to /var/lib/timezone/localtime
Update /etc/timezone in enter-chroot, by guessing its value from
the current /etc/localtime.
Add -T to the creation of /etc/localtime symlink, in the unlikely
case where /etc/localtime is a directory.
Revert "Update /etc/timezone in enter-chroot, by guessing its value from"

After discussion, updating /etc/timezone is not necessary, and even
possibly harmful, as Ubuntu would erase /etc/localtime upon tzdata
upgrade if /etc/timezone exists.

This reverts commit 94566f369f2e34ca2688b8584ff3c6955561b131.
Document why we are removing /etc/zoneinfo from the guest OS.
Check that the link /etc/localtime points to is a valid timezone in the guest OS.
Simplify timezone sanity check.
ChromeOS -> Chromium OS