The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/6a58a510c0c29a21bce45e9af8097eaad543ce44

commit 6a58a510c0c29a21bce45e9af8097eaad543ce44
Author: Chan <chanli@chromium.org>
Date: Tue Sep 11 21:31:46 2018

[Findit] Flake: Add last_occurred_time to Flake model.

This field is to record the timestamp of the most recent occurrence of a flake.

Bug:  881622 
Change-Id: Ie3f1163ccee804f23297fa2aa94f5bdeae3345f2
Reviewed-on: https://chromium-review.googlesource.com/1216751
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Chan Li <chanli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#17510}
[modify] https://crrev.com/6a58a510c0c29a21bce45e9af8097eaad543ce44/appengine/findit/services/flake_detection/test/detect_cq_false_rejection_flakes_test.py
[modify] https://crrev.com/6a58a510c0c29a21bce45e9af8097eaad543ce44/appengine/findit/model/flake/detection/flake_occurrence.py
[modify] https://crrev.com/6a58a510c0c29a21bce45e9af8097eaad543ce44/appengine/findit/model/flake/flake.py
[modify] https://crrev.com/6a58a510c0c29a21bce45e9af8097eaad543ce44/appengine/findit/services/flake_detection/detect_cq_false_rejection_flakes.py
[modify] https://crrev.com/6a58a510c0c29a21bce45e9af8097eaad543ce44/appengine/findit/handlers/flake/detection/test/flake_detection_utils_test.py