OptionalcontainerOptionalheader_The empty strip above the menu bar, where a host puts its logo. 57px by default.
false removes it and everything below moves up; a number sets its height in pixels.
Omit it and the layout is exactly what it has always been — the stylesheet keeps 57px as
its fallback, so a page that says nothing is untouched.
Where the chrome is built. Defaults to document.body.