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According to UX spec, labels in FeaturePodButton should show while hover
background. In order to implement this, this CL adds
FeaturePodLabelButton class. Also, this CL fixes FeaturePodIconButton
padding which was different from UX spec.

According to the spec, icon and labels in feature pod button are
different click targets, which is still not implemented in this CL. It
will be done in another CL.

Screencast: http://dr/file/d/1FPylXzoO_xCTc8ywQU2NezUug9WgTz-q/view
UX spec: http://shortn/_U7QSE9nhSk

TEST=manual
BUG=821671

Change-Id: I596abbf5226932c9ac4f342e29c83fa4cc12185d
Reviewed-on: https://chromium-review.googlesource.com/1008942Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550068}