Callers never specify this argument. It defaults to nsnull in the header.
Created attachment 228393 [details] [diff] [review] remove the argument Seems pretty straightforward, but there is a bit of a ripple effect into some implementation functions. I tested this on Windows, with and without Cairo. Don't have access to other platforms. I didn't check other functions on nsIRenderingContext which have an aFontID argument. I'm only familiar with GetBoundingMetrics().
nsRenderingContext::GetBoundingMetrics no longer has an aFontID argument.