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This makes sure that we only allow up to 30 scheduled notifications and
handles the case when we update an existing notification with a
scheduled one.

Bug: 891339
Change-Id: Ifb884367d71f9156abf303d8330364f671e3afb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505503
Commit-Queue: Richard Knoll <knollr@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638686}