Change viewport styling to more closely match v4
Change viewport styling to more closely match v4

Styling for addon-viewport iframe was a 10px black border which was a
little intrusive when working on stories. This commit changes the iframe
style to a more familiar style that mimics v4 with horizontal centering
and a subtle box-shadow.

storybooks/storybook,storybooks/storybook,kadirahq/react-storybook,storybooks/storybook,storybooks/react-storybook,kadirahq/react-storybook,storybooks/storybook,storybooks/react-storybook,storybooks/storybook,storybooks/storybook,storybooks/react-storybook,storybooks/react-storybook,storybooks/storybook