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>
diff --git a/chroot_version_hooks.d/141_unmerge_libchrome b/chroot_version_hooks.d/141_unmerge_libchrome
new file mode 120000
index 0000000..b93eefc
--- /dev/null
+++ b/chroot_version_hooks.d/141_unmerge_libchrome
@@ -0,0 +1 @@
+139_unmerge_libchrome
\ No newline at end of file