Fix run-gtests.sh for device.

Test: run-gtests.sh
Change-Id: I6d747e4b95f172e5975508e65f4a64c1d25d0d56
diff --git a/tools/run-gtests.sh b/tools/run-gtests.sh
index 0f333f6..796d594f 100755
--- a/tools/run-gtests.sh
+++ b/tools/run-gtests.sh
@@ -60,7 +60,7 @@
 options="$@"
 
 run_in_chroot() {
-  if [ -n $ART_TEST_ON_VM ]; then
+  if [ -n "$ART_TEST_ON_VM" ]; then
     $ART_SSH_CMD $ART_CHROOT_CMD $@
   else
     "$adb" shell chroot "$ART_TEST_CHROOT" $@