set_chronos_password.sh: Add missing sudo while grepping the contents of shadow on root fs.

BUG=none
TEST=manual

Change-Id: Ie0d237c0dc6a2c45ffc33b2fcc22279127854ed9

Review URL: http://codereview.chromium.org/3146025
diff --git a/scripts/image_signing/set_chronos_password.sh b/scripts/image_signing/set_chronos_password.sh
index f1b1ac3..ff71e45 100755
--- a/scripts/image_signing/set_chronos_password.sh
+++ b/scripts/image_signing/set_chronos_password.sh
@@ -19,7 +19,7 @@
   local temp_shadow="$rootfs/etc/tempshadow"
   echo "chronos:$crypted_password:14500:0:99999::::" \
     | sudo tee "$temp_shadow" > /dev/null
-  grep -Ev ^chronos: "$rootfs/etc/shadow" \
+  sudo grep -Ev ^chronos: "$rootfs/etc/shadow" \
     | sudo tee -a "$temp_shadow" > /dev/null
   sudo mv -f "$temp_shadow" "$rootfs/etc/shadow"
 }