[Swarming] Let test runner exit gracefully after test failures.

tools/run-tests.py

 #!/usr/bin/env python
 #
 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 770 matching lines @@
 
   exit_code = runner.Run(options.j)
   overall_duration = time.time() - start_time
 
   if options.time:
     verbose.PrintTestDurations(suites, overall_duration)
 
   if num_tests == 0:
     print("Warning: no tests were run!")
 
+  if exit_code == 1 and options.json_test_results:
+    print("Force exit code 0 after failures. Json test results file generated "
+          "with failure information.")
+    exit_code = 0
+
   return exit_code
 
 
 if __name__ == "__main__":
   sys.exit(Main())