Update the libmetrics API to match the new Chrome interface.

src/platform/init/chromeos_shutdown

Index: src/platform/init/chromeos_shutdown
diff --git a/src/platform/init/chromeos_shutdown b/src/platform/init/chromeos_shutdown
index c4e2d8af4065fdc06bda9b0cf154ed8bef2df7b6..fe06844e211c657d822deb16f5c864471bf08f95 100755
--- a/src/platform/init/chromeos_shutdown
+++ b/src/platform/init/chromeos_shutdown
@@ -7,11 +7,7 @@
 # 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