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Lazy creation meant that sometimes, at least in tests,
UserScriptListener wasn't created early enough to do its job (profiles
were added and user script update notifications were sent before
UserScriptListener was created).

Bug: none
Change-Id: Id0a3c394da2d5bd2c58da5d26118f4448c68107d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1853912
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708079}