Fixed centered-alignment if dialog is taller than the viewport.
Fixed centered-alignment if dialog is taller than the viewport.

The spec dictates that if the dialog is displayed in centered-alignment
and the dialog is taller than the viewport, then the dialog shall be placed
at the top of the viewport.  Instead, the polyfill was always centering the
dialog even if it was taller than the viewport.

elektronik2k5/dialog-polyfill,lozandier/dialog-polyfill,samthor/dialog-polyfill,huangxok/dialog-polyfill,GoogleChrome/dialog-polyfill,GoogleChrome/dialog-polyfill,lozandier/dialog-polyfill,elektronik2k5/dialog-polyfill,samthor/dialog-polyfill,huangxok/dialog-polyfill