<dialog>: Non-modals are no longer magically positioned

As per spec change: http://html5.org/r/8457
Now <dialog> is centered and promoted to the top layer only in showModal().

Existing tests are modified to meet the change. In
particular I revamped non-anchored-dialog-positioning.html
not just for the new change but also to be more readable.

BUG=332133

Review URL: https://codereview.chromium.org/130183005

git-svn-id: svn://svn.chromium.org/blink/trunk@166900 bbb929c8-8fbe-4397-9dbb-9b2b20218538