Update the libmetrics API to match the new Chrome interface.

src/platform/init/chromeos_shutdown

 #!/bin/sh
 
 # Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Shutdown is best-effort. We don't want to die on errors.
 set +e
 
-# Measure shutdown time
-if [ ! -d "/var/log/metrics" ]; then
-    mkdir -p /var/log/metrics
-fi
-
+# Measure shutdown time. /var/log/metrics is created by chromeos_startup.
 cut -f 1 -d ' ' /proc/uptime > /var/log/metrics/uptime_shutdown_start
 cat /sys/block/sda/stat > /var/log/metrics/disk_shutdown_start
 
 # Sync the system clock to the hardware clock.
 HWCLOCK=/sbin/hwclock
 ${HWCLOCK} --systohc --utc --noadjfile &
 
 . /sbin/killers
 
 # Remount root in case a developer has remounted it rw for some reason.
@@ skipping 26 matching lines @@
 if [ $? -ne 0 ] ; then
   mount > /mnt/stateful_partition/var/log/shutdown_stateful_umount_failure
   mount > /var/log/shutdown_stateful_umount_failure
 fi
 
 # Just in case something didn't unmount properly above.
 sync
 
 # Ensure that we always claim success.
 exit 0