Change <dialog> centering to not mutate style.

Previously, <dialog> centering was implemented via
customStyleForRenderer. However, it's non-conforming to adjust the
computed style; the spec mandates that only the static position is
changed. This patch changes centering to occur during layout of
RenderView, which is the parent of all absolutely positioned dialogs.

BUG=232268

Review URL: https://codereview.chromium.org/26667002

git-svn-id: svn://svn.chromium.org/blink/trunk@159767 bbb929c8-8fbe-4397-9dbb-9b2b20218538