This MXR search...
...shows that GetPatternFirstChild() is only mentioned inside of nsSVGPatternFrame.[h,cpp]
This currently has NS_IMETHOD / NS_IMETHODIMP annotations, but these aren't necessary, because the method isn't declared in an IDL file.
This method can just return nsresult.
BTW, hg/cvs archeology shows that the line of code with this method's declaration hasn't changed since the original revision of the file, from bug 336653 ("Remove nsISVGPattern"). So, presumably this is just left over from when we had an XPCOM nsISVGPattern interface.
Actually, this method can just directly return a nsIFrame*.
Created attachment 569730 [details] [diff] [review]