PRESUBMIT.cfg: improve PRESUBMIT.py check.

Switch to using presubmit_support.py for the delegation out of
PRESUBMIT.cfg, doesn't rely on the string parsing of the old approach.

BUG=chromium:1039875
TEST=generate

Change-Id: Ib28a6edcad3138e7b2a5821fd063fd6a200e7663
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2107560
Reviewed-by: David Burger <dburger@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Tested-by: LaMont Jones <lamontjones@chromium.org>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index 11adbc6..2b99975 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -1,5 +1,5 @@
 [Hook Scripts]
-git_cl_presubmit = if git cl presubmit -v | grep Error; then exit 1; fi
+git_cl_presubmit = presubmit_support.py
 
 [Hook Overrides]
 # The only Golang code here is from protoc.