WebVR: lock focus while presenting to presenting window

chrome/test/data/android/webvr_instrumentation/resources/webvr_boilerplate.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var webglCanvas = document.getElementById("webgl-canvas");
 var glAttribs = {
   alpha: false,
 };
 var gl = webglCanvas.getContext("webgl", glAttribs);
 var vrDisplay = null;
 var frameData = null;
+var onAnimationFrameCallback = null;
 
 function onResize() {
   if (vrDisplay && vrDisplay.isPresenting) {
     var leftEye = vrDisplay.getEyeParameters("left");
     var rightEye = vrDisplay.getEyeParameters("right");
 
     webglCanvas.width = Math.max(leftEye.renderWidth, rightEye.renderWidth) * 2;
     webglCanvas.height = Math.max(leftEye.renderHeight, rightEye.renderHeight);
   } else {
     webglCanvas.width = webglCanvas.offsetWidth * window.devicePixelRatio;
@@ skipping 11 matching lines @@
 
 function onAnimationFrame(t) {
   if (vrDisplay == null) {
     window.requestAnimationFrame(onAnimationFrame);
     gl.viewport(0, 0, webglCanvas.width, webglCanvas.height);
     return;
   }
   vrDisplay.requestAnimationFrame(onAnimationFrame);
   // If presenting, set canvas to blue. Otherwise, red.
   if (vrDisplay.isPresenting) {
+    if (onAnimationFrameCallback) onAnimationFrameCallback();
     vrDisplay.getFrameData(frameData);
 
     gl.clearColor(0.0, 0.0, 1.0, 1.0);
     gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
 
     gl.viewport(0, 0, webglCanvas.width * 0.5, webglCanvas.height);
     gl.viewport(webglCanvas.width * 0.5, 0, webglCanvas.width * 0.5,
                 webglCanvas.height);
 
     vrDisplay.submitFrame();
@@ skipping 20 matching lines @@
 
 gl.clearColor(1.0, 0.0, 0.0, 1.0);
 gl.enable(gl.DEPTH_TEST);
 gl.enable(gl.CULL_FACE);
 window.addEventListener("resize", onResize, false);
 window.addEventListener("vrdisplaypresentchange", onVrPresentChange, false);
 window.addEventListener('vrdisplayactivate', onVrRequestPresent, false);
 window.requestAnimationFrame(onAnimationFrame);
 webglCanvas.onclick = onVrRequestPresent;
 onResize();