chromeos: Minor visual tweaks to keyboard overlay.

This adds more padding around the labels in the keyboard
overlay and makes shortcut labels stand out when they're
displayed on top of keycap labels.

BUG=157255
R=mazda@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/26734005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227948 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227948