[builder_alerts] Send failed step names to Findit.
[builder_alerts] Send failed step names to Findit.

This is for incremental analysis of a build failure when there are new failed steps.

Currently, Findit pulls alerts data from Sheriff-o-Matic periodically and checks for new failed steps, because builder_alerts only pushes info of failed builds (master_url, bulider_name, build_number) to Findit.
This change is to switch to also pushing info of failed steps from builder_alerts to Findit.

Because huge alerts data (~30MB) from SoM recently causes two problems on Findit side:
1. Parsing the huge data leads to sudden and sharp increase of memory, results in Out of Memory, and Findit app instances restarts frequently.
   Increasing the memory for the app instances could reduce the chance of OOM, but it is not elegant.
2. Pulling the huge data over HTTP requests takes too long, and results in frequent DeadlineExceededError.

BUG=501021

Review URL: https://codereview.chromium.org/1189873002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra