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This CL adds a basic integration framework to allow writing integration
tests for trigger scripts. This is done analogous to the existing way
for regular scripts. However, the new way of injecting the request
sender directly (rather than a test service) is much more generic and
should in the mid-term replace the older way.

This CL also fixes several small issues that surfaced during testing:
- Fixed issue where an empty callout message would be shown.
- Fixed issue where the first time user flag was set too early.
- Fixed issue where the Facade would not correctly trigger the new
trigger scripts.

Bug: b/171776026
Change-Id: I7c5a77cd317996d58db57026e322fd982dfacdf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2524530
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Marian Fechete <marianfe@google.com>
Cr-Commit-Position: refs/heads/master@{#826260}