Center dialogs before computing overflow.

Otherwise, the RenderView's layoutOverflowRect() is computed as if the dialog
were positioned inline.

BUG=403136

Review URL: https://codereview.chromium.org/574143002

git-svn-id: svn://svn.chromium.org/blink/trunk@182130 bbb929c8-8fbe-4397-9dbb-9b2b20218538