Skia does some nasty things that break with DLL calls like having virtual dtors
without a body definition (and it's honestly surprising that it even builds to
begin with the way it's done, but apparently they use some linking symbol voodoo
to work around that in dependent classes).
Not wanting to have to pull Google code apart to try and fix that for something
we're only using for canvas anyway, it's better to just let Skia reside in
libxul instead.
