Center fixed positioned <dialog>

Now fixed positioned dialogs with auto 'top' and 'bottom' are
centered in the viewport, like abspos dialogs. Previously, the dialog
would simply not display.

There are two failing tests in fixpos-dialog-layout.html. There is
still a bug where a <dialog> with auto 'top' and 'bottom' that was
*not* centered (an unusual case) is not displayed.

BUG=382594

Review URL: https://codereview.chromium.org/342943002

git-svn-id: svn://svn.chromium.org/blink/trunk@176590 bbb929c8-8fbe-4397-9dbb-9b2b20218538