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According to UX, in addition to expanding the latest notification,
older notification popups should be collapsed.

See the screencast:
- Without this CL:
  http://go/scrcast/NTY3OTAwNjEyOTA2MTg4OHxjMjY3MGMxMS0xNw
- With this CL: http://go/scrcast/NTY0MjAyMzc3NjI4ODc2OHxhYjBmNWQ4Yy02NA

TEST=NotificationViewMDTest.ExpandLongMessage
BUG=792582

Change-Id: Ie6693386acda5a47405350822a48b4b0a279029f
Reviewed-on: https://chromium-review.googlesource.com/830998Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525034}