Fix chromeos-common.sh: false -> ${FLAGS_FALSE}

Change-Id: Ic1b0765470dcba081109adf7a34318fb4a4ac7c8

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6777012
diff --git a/chromeos-common.sh b/chromeos-common.sh
index 061a775..b2b45b6 100644
--- a/chromeos-common.sh
+++ b/chromeos-common.sh
@@ -126,7 +126,7 @@
   local force_full="${6:-}"
   local rootfs_default_size=2048  # 2GiB
   local rootfs_size="${7:-$rootfs_default_size}"
-  local large_test_partitions="${8:-false}"
+  local large_test_partitions="${8:-${FLAGS_FALSE}}"
 
   if [ "$rootfs_size" = "default" ]; then
     rootfs_size=$rootfs_default_size