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: Ieefdce5f843d5602c2336c6fc5514a513cc0e2e0
Reviewed-on: https://chromium-review.googlesource.com/260751
Reviewed-by: Steve Fung <stevefung@chromium.org>
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index 51d8dd6..43d092e 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -2,6 +2,9 @@
 # Comment out the disable-flags for any checks you want to leave enabled.
 
 [Hook Overrides]
+# Make sure Manifest files are valid.
+manifest_check: true
+
 stray_whitespace_check: false
 long_line_check: false
 cros_license_check: false