Trouble getting the watermark to work

[hillelcoren]

When trying to add the watermark property, for example:

var dd = {
    content: [
        'First paragraph',
    ],
    watermark: 'test'
}

I get the following error:

Uncaught Error: Font 'undefined' in style 'normal' is not defined in the font section of the document definition.

This is both in my dev environment and when trying it in the playground.

[liborm85]

Fixed by #492