Add repo whitelist for pre-cq exonerator.

BUG=chromium:793042
TEST=None

Change-Id: I480bcd7d68bb8602def760107d05c4404e05dfb2
Reviewed-on: https://chromium-review.googlesource.com/936682
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Paul Hobbs <phobbs@google.com>
Reviewed-by: Paul Hobbs <phobbs@google.com>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/exonerator/gerrit_precq.py b/exonerator/gerrit_precq.py
index e12acb7..d6befe7 100644
--- a/exonerator/gerrit_precq.py
+++ b/exonerator/gerrit_precq.py
@@ -19,6 +19,8 @@
 CL-Exonerator has triggered a Pre-CQ retry on this CL due to a Pre-CQ sanity
 failure.
 """
+_REPOSITORY_WHITELIST = frozenset(['infra/dummies/general-sandbox'])
+
 
 def MaybeExonerate(change, build_id):
   """Mark a CL as Trybot-Ready only if it hasn't already been.
@@ -38,6 +40,11 @@
         "Change %s was not ready to exonerate %d", str(change), build_id)
     return False
 
+  if not details['project'] in _REPOSITORY_WHITELIST:
+    logging.info(
+        'Change %s has project %s, which is not whitelisted.',
+        details, details['project'])
+
   try:
     helper.SetReview(
         change.gerrit_number,