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This patch implements the following:
- auto-dismiss notification issues after 1 minute
- auto-dismiss non-notification non-blocking issue after 5 minutes

This is done by posting a task to clear a given issue after it has been
added to the IssueManager.

Part 2 will be to dismiss non-blocking issues following a successful
cast from the Media Router UI.

Bug: 763432
Change-Id: I6c55d4c272b8fc09b48935c6d4011d893e0cea58
Reviewed-on: https://chromium-review.googlesource.com/658138
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501334}