Use delta_generator from chroot instead of from update_engine directory.

cros_generate_update_payload

Index: cros_generate_update_payload
diff --git a/cros_generate_update_payload b/cros_generate_update_payload
index 8203d84fd777b8594c08790713b66323232cd097..829f7a3a45d2a8e438877a66e08c7bd5a4132c72 100755
--- a/cros_generate_update_payload
+++ b/cros_generate_update_payload
@@ -168,8 +168,8 @@ if [ "$DELTA" -eq "$FLAGS_TRUE" -o "$FLAGS_old_style" -eq "$FLAGS_FALSE" ]; then
   echo "Generating a delta update"
 
   # Sanity check that the real generator exists:
-  GENERATOR="$(dirname "$0")/../platform/update_engine/delta_generator"
-  [ -x "$GENERATOR" ] || die "$GENERATOR doesn't exist, or isn't executable"
+  GENERATOR="$(which delta_generator)"
+  [ -x "$GENERATOR" ] || die "can't find delta_generator"
 
   trap cleanup INT TERM EXIT
   if [ "$DELTA" -eq "$FLAGS_TRUE" ]; then