Twenty Thirteen: ensure galleries are displayed correctly (centered) on both single and indexed views, props obenland. See #24307.

git-svn-id: https://core.svn.wordpress.org/trunk@24285 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: fix pixel shift that can occur on gallery non-single views in latest Chrome. Props jeremyfelt, fixes #24310.

git-svn-id: https://core.svn.wordpress.org/trunk@24286 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: revert unintended space change from r24239.

git-svn-id: https://core.svn.wordpress.org/trunk@24287 1a063a9b-81f0-0310-95a4-ce76da25c4cd