The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66

commit 47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66
Author: Chan <chanli@chromium.org>
Date: Tue Nov 20 22:09:33 2018

[Findit] Add merge_destination_key to FlakeIssue

Add merge_destination_key to FlakeIssue to store the FINAL merge desination of a flake issue. If FlakeIssueA -> FlakeIssueB -> FlakeIssueC, both A and B will store C's key to their merge_destination_key

Also in this change:
1. Use monorail_project and issue_id to construct the key to a FlakeIssue entity since we don't need to store multiple entities with the same issue_id any more
  a. Current data need to be cleaned up, a script in Cl https://chromium-review.googlesource.com/c/infra/infra/+/1337374 will take care of it, and I'll run the script after new deployment
2. When taking action on FlakeIssues, always check merge destination issue first, and fall back to associated issue if the issue is not merged into any other one

Bug:  905450 
Change-Id: I5b6c6ce56b1f3931e3cedc4b11a99d433e4191b4
Reviewed-on: https://chromium-review.googlesource.com/c/1336172
Commit-Queue: Chan Li <chanli@chromium.org>
Reviewed-by: Jeffrey Li <lijeffrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#19101}
[modify] https://crrev.com/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66/appengine/findit/model/flake/test/flake_issue_test.py
[modify] https://crrev.com/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66/appengine/findit/services/test/flake_issue_util_test.py
[modify] https://crrev.com/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66/appengine/findit/handlers/flake/detection/rank_flakes.py
[modify] https://crrev.com/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66/appengine/findit/model/flake/flake_issue.py
[modify] https://crrev.com/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66/appengine/findit/handlers/flake/detection/flake_detection_utils.py
[modify] https://crrev.com/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66/appengine/findit/services/flake_issue_util.py
[modify] https://crrev.com/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66/appengine/findit/handlers/flake/detection/test/show_flake_test.py
[modify] https://crrev.com/47dd7e3ff5dc2f268f7e973eb7eb0d822d732d66/appengine/findit/handlers/flake/detection/test/flake_detection_utils_test.py