create fabric canvas by DOM object instead of id in order to enable the SketchFieldTest
create fabric canvas by DOM object instead of id in order to enable the SketchFieldTest

The binding between fabric.document and window.document (in fabricjs source code) would be broken in Phantom or Chrome test environment, and thus fabric could not create canvas object by id.

tbolis/react-sketch,tbolis/react-sketch,tbolis/react-sketch