presubmit: enable Manifest check

We're moving the hardcoded check lists out of the pre-upload script.

BUG=chromium:466264
TEST=uploading a CL w/a bad Manifest is rejected
CQ-DEPEND=CL:262042

Change-Id: Ic3ced6d18c3c64157ae14b669868aa28965bd0d3
Reviewed-on: https://chromium-review.googlesource.com/260733
Reviewed-by: Steve Fung <stevefung@chromium.org>
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
new file mode 100644
index 0000000..824826d
--- /dev/null
+++ b/PRESUBMIT.cfg
@@ -0,0 +1,3 @@
+[Hook Overrides]
+# Make sure Manifest files are valid.
+manifest_check: true