WebXRManager: Added getReferenceSpace() method.

src/renderers/webxr/WebXRManager.js

@@ -126,6 +126,12 @@ function WebXRManager( renderer, gl ) {
 
 	};
 
+	this.getReferenceSpace = function () {
+
+		return referenceSpace;
+
+	};
+
 	this.getSession = function () {
 
 		return session;