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We should be careful to not let the same download completion update
surface twice to the UI layer. Otherwise it might inadvertently
introduce duplicate notification bugs.

Bug : 949941

Change-Id: I161066f81fd6d12afb57236847d9d6c47fb2984a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1555653
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649340}