Skip presubmits when doing --download-data-only.

BUG=v8:4124
LOG=N
R=machenbach@chromium.org
TEST=None

Review URL: https://codereview.chromium.org/1148093002

Cr-Commit-Position: refs/heads/master@{#28527}

tools/run-tests.py

@@ -343,6 +343,8 @@ def ProcessOptions(options):
     # Buildbots run presubmit tests as a separate step.
     options.no_presubmit = True
     options.no_network = True
+  if options.download_data_only:
+    options.no_presubmit = True
   if options.command_prefix:
     print("Specifying --command-prefix disables network distribution, "
           "running tests locally.")