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This CL adds the basic file structure for trigger scripts. Most classes
are only stubs, but the TriggerScript condition checking is already in
place. Subsequent CLs will replace the stubs to make this fully
functional.

Bug: b/171776026
Change-Id: Ic6b687242048c3f6ebf665d7611ff115a9a91ccd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513595
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Marian Fechete <marianfe@google.com>
Cr-Commit-Position: refs/heads/master@{#823523}