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

tools/run-tests.py

Index: tools/run-tests.py
diff --git a/tools/run-tests.py b/tools/run-tests.py
index 7e3587989bb70e56454257c3487487fa6739ab22..e8c1b4d884b9e43d24a48dd018ccf262182335ba 100755
--- a/tools/run-tests.py
+++ b/tools/run-tests.py
@@ -788,6 +788,11 @@ def Execute(arch, mode, args, options, suites):
   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