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For better MD style download notification representation, we're
going to add new attributes to Notification class, and going to
use these strings.

TEST=manual
BUG=740807

Change-Id: I05a5f1a2094814b579aee886297732b0df2c56b5
Reviewed-on: https://chromium-review.googlesource.com/578752Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488184}