This IDL change is needed for webaudio's WaveShaperNode, where a
null BufferSource for a curve attribute results in a zero-length buffer.
WebGL also has a nullable BufferSource arg in bufferData(...). But
there, a null data/srcData value returns GL_INVALID_VALUE.
This is more like what the IDL files specify with two different mixins,
but the inheritance structure here is slightly different for easier
maintenance. This will also allow the WebGL2 Impl to inherit from the
WebGL1 Impl as WebGL versions don't share the functions defined in the
Overloads interfaces.
