WebGLBackground: More robust getSession() code.

src/renderers/webgl/WebGLBackground.js

@@ -30,7 +30,8 @@ function WebGLBackground( renderer, state, objects, premultipliedAlpha ) {
 		// Ignore background in AR
 		// TODO: Reconsider this.
 
-		var session = renderer.vr.getSession();
+		var vr = renderer.vr;
+		var session = vr.getSession && vr.getSession();
 
 		if ( session && session.environmentBlendMode === 'additive' ) {