Set nacl_helper as executable.  It is now being used, and NaCl needs it +x

A chrome change:
https://src.chromium.org/viewvc/chrome?view=rev&revision=212997
Made nacl_helper be used.  This works on linux, but not chromeos.

BUG=chromium:263525
TEST=none

Change-Id: I36258bb21a0012b7625f579d00794ce37341f498
Previous-Reviewed-on: https://gerrit.chromium.org/gerrit/63224
(cherry picked from commit 93281fa6a9fa1af48aae02d0a9c142a4450af51e)
Reviewed-on: https://gerrit.chromium.org/gerrit/63285
Reviewed-by: Josafat Garcia <josafat@chromium.org>
Tested-by: Josafat Garcia <josafat@chromium.org>
Commit-Queue: Jan Voung <jvoung@chromium.org>
Tested-by: Jan Voung <jvoung@chromium.org>
diff --git a/lib/chrome_util.py b/lib/chrome_util.py
index a03b7a7..5a9b30b 100644
--- a/lib/chrome_util.py
+++ b/lib/chrome_util.py
@@ -359,6 +359,7 @@
   Path('nacl_irt_*.nexe',
        cond=C.GypNotSet(_DISABLE_NACL)),
   Path('nacl_helper',
+       exe=True,
        optional=True,
        cond=C.GypNotSet(_DISABLE_NACL)),
   Path('resources/'),