Enable cros_download_vm flag for betty-pi-arc in DEPS

Without this flag, VM image for betty-pi-arc is not downloaded when
gclient runhooks is run

R=bpastene@chromium.org

Bug: 1000513
Change-Id: Ice17757549dd3a4ca506a6eec5e8e5f487425cda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815882Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Anushruth  <anushruth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698609}

DEPS

@@ -128,7 +128,7 @@ vars = {
   'checkout_simplechrome': '(checkout_chromeos and host_os == "linux") and ("{cros_board}" != "")',
   # Surround the board var in quotes so gclient doesn't try parsing the string
   # as an expression.
-  'cros_download_vm': '("{cros_board}" == "amd64-generic") or ("{cros_board}" == "betty")',
+  'cros_download_vm': '("{cros_board}" == "amd64-generic") or ("{cros_board}" == "betty") or ("{cros_board}" == "betty-pi-arc")',
   # Should we build and test for public (ie: full) CrOS images, or private
   # (ie: release) images.
   'use_public_cros_config': 'not checkout_src_internal',