[canvas] integration with WebIDL

All,

the canvas specification recently moved to WebIDL for its function signatures.
Even though there should be no change in behavior, I can't find if this change was discussed or approved by the group.

Does anyone know?
Adding WebIDL to the interface fixed the issue that the canvas IDL didn't match the behavior (i.e. if you set globablAlpha with NaN, you are not supposed to get a type error)

Rik