Allow style sharing for <dialog>.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@159871 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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