[content/test] Ensure the return code of run_gpu_integration_test is …

…non-zero if test fails

R=kbr@chromium.org, bajones@chromium.org, zmo@chromium.org
BUG=352807
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2077213002
Cr-Commit-Position: refs/heads/master@{#401200}

content/test/gpu/run_gpu_integration_test.py

@@ -40,6 +40,7 @@ def main():
   option, _ = parser.parse_known_args(rest_args)
   if option.write_abbreviated_json_results_to:
     PostprocessJSON(option.write_abbreviated_json_results_to)
+  return retval
 
 if __name__ == '__main__':
   sys.exit(main())