chroot hook: Unmerge old libchrome when switching to new slot

libchrome is being uprevved to r395517

BUG=b:28985443
TEST=./build_packages
CQ-DEPEND=CL:346457

Change-Id: Ib544e025523332818832d452773670020839b386
Reviewed-on: https://chromium-review.googlesource.com/349101
Commit-Ready: Heng-ruey Hsu <henryhsu@google.com>
Tested-by: Heng-ruey Hsu <henryhsu@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed