GitLab

Previously tombstones are saved as a long string test result object.
Tombstones are usually huge, so we have decided to write tombstones to
logdog and save the url, instead of the whole tombstones, as part of
test result object.

BUG=605572

Review-Url: https://codereview.chromium.org/2577463003
Cr-Commit-Position: refs/heads/master@{#438665}