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This change updates the close button style. The color of the 'x' is now
white. There is no background unless the user hovers over the icon. In
this case, there is a grey circular background.

As part of this change, CloseIconButton is created to account for the
hover behavior.

Bug: 872098
Change-Id: I90a53c5e42d6377db4ee45b47633689315865c18
Reviewed-on: https://chromium-review.googlesource.com/1168732
Commit-Queue: apacible <apacible@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582271}