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Needed because default notification close button is part of views
hierarchy and covered by the notification surface. This CL adds
a floating version of close button that sits on top of the notification
surface.

BUG=b/25816173
TBR=brettw@chromium.org

Review-Url: https://codereview.chromium.org/2093563007
Cr-Commit-Position: refs/heads/master@{#403196}