Add disabled platforms column to go/csh-stories

- PopulateExpectations accepts a dictionary:
    all_expectations = {story_name: [[conditions], reason]}
  Returns disables dictionary:
    disables = {story_name: "Disabled Platforms"}

- generate_system_health_csv_unittest.py tests the new
PopulateExpectations function.

Bug: chromium:723636
Change-Id: Ic94b08f53c004bab5aa813399da34dd7d4b09279
Reviewed-on: https://chromium-review.googlesource.com/674099
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Lori Oliver <loloangela@google.com>
Cr-Commit-Position: refs/heads/master@{#504729}
3 files changed