Allow style sharing for <dialog>.

r151832 disabled style sharing for <dialog>, but that's no longer
necessary since after r159767 dialog doesn't depend on style for
centering.

No behavior change, covered by existing test
multiple-centered-dialogs.html

BUG=232268

Review URL: https://codereview.chromium.org/27197014

git-svn-id: svn://svn.chromium.org/blink/trunk@159871 bbb929c8-8fbe-4397-9dbb-9b2b20218538