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According to the spec, new shelf should have 8dip padding between the
top of the shelf and the bottom of the popup notifications.

TEST=manual
BUG=883640

Change-Id: I6186379d221624b76984af69b922f85e36556401
Reviewed-on: https://chromium-review.googlesource.com/1250344Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595468}