Start WebVRManager animation when setting the animation callback

src/renderers/webvr/WebVRManager.js

@@ -379,6 +379,8 @@ function WebVRManager( renderer ) {
 
 		animation.setAnimationLoop( callback );
 
+		if ( isPresenting() ) { animation.start(); }
+
 	};
 
 	this.submitFrame = function () {