For determining OWNERS, drop reviewers specified via '-r' that aren't in email address format.

presubmit_canned_checks.py

Index: presubmit_canned_checks.py
diff --git a/presubmit_canned_checks.py b/presubmit_canned_checks.py
index a48a0c627de361c6927477c7734bf2b7ab29a543..878ee70a68ff71b12bed82dd3082a90cae7219fd 100644
--- a/presubmit_canned_checks.py
+++ b/presubmit_canned_checks.py
@@ -859,7 +859,9 @@ def _ReviewersFromChange(change):
     reviewers.update(set([r.strip() for r in change.R.split(',')]))
   if change.TBR:
     reviewers.update(set([r.strip() for r in change.TBR.split(',')]))
-  return reviewers
+
+  # Drop reviewers that aren't specified in email address format.
+  return set(reviewer for reviewer in reviewers if '@' in reviewer)
 
 
 def _RietveldOwnerAndReviewers(input_api, email_regexp, approval_needed=False):