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This change involves two semantic differences:
1. stable-gitiles-trigger is a poller rather than a scheduled trigger
because it's not possible to generate a scheduled trigger using
lucicfg. With the upcoming branch, this is a change that would need to
be made soon regardless.
2. The notifiers for builders that are scheduled by a trigger now
populate the repository field.

Bug: 1011908
Change-Id: I217089efba52bd6ec92b291f2141a6e36e1477d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031189
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736968}