Note this only happens when one of the shards has a swarming error. Since json test results are still being uploaded, the link is still there, swarming just errors out before it adds the links from the json: https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/swarming/api.py?l=951

I am proposing to move this addition to before the _handle_summary_json call so the data gets added regardless.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/b7d4442614306d82585c17f9ba050b622b9d4450

commit b7d4442614306d82585c17f9ba050b622b9d4450
Author: Emily Hanley <eyaich@google.com>
Date: Wed May 16 11:13:59 2018

Update logic to write out buildbot links when there is a swarming error.

Bug:842680
Change-Id: I67c3d21bf3af05a0c98a3211b95761bdb0f270b8
Reviewed-on: https://chromium-review.googlesource.com/1057363
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Emily Hanley <eyaich@chromium.org>

[modify] https://crrev.com/b7d4442614306d82585c17f9ba050b622b9d4450/scripts/slave/recipes/chromium.expected/isolated_script_test_custom_merge_script_with_args.json
[modify] https://crrev.com/b7d4442614306d82585c17f9ba050b622b9d4450/scripts/slave/recipes/chromium.expected/isolated_script_test_custom_merge_script.json
[modify] https://crrev.com/b7d4442614306d82585c17f9ba050b622b9d4450/scripts/slave/recipe_modules/chromium_tests/tests/steps/swarming_isolated_script_test.expected/chartjson_no_results_failure.json
[modify] https://crrev.com/b7d4442614306d82585c17f9ba050b622b9d4450/scripts/slave/recipe_modules/swarming/api.py
[modify] https://crrev.com/b7d4442614306d82585c17f9ba050b622b9d4450/scripts/slave/README.recipes.md
[modify] https://crrev.com/b7d4442614306d82585c17f9ba050b622b9d4450/scripts/slave/recipes/chromium.expected/isolated_script_test_custom_results_handler.json