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- Crash was due to attempting to remove an previously removed
  notification.
- The fix is to verify that the notification_id is a valid id to remove.

Bug: 1139561, 1106937
Test: chromeos_components_unittests, ash_unittests
Change-Id: Ia606df1dfec13beba9ccc14c2618ec4f6fda7129
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2482428Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Jimmy Gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818372}