Better generalize VR test framework

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.
 
+// Add additional setup steps to the object from webvr_e2e.js if it exists.
+if (typeof initializationSteps !== "undefined") {
+  initializationSteps["getVRDisplays"] = false;
+} else {
+  // Create here if it doesn't exist so we can access it later without checking
+  // if it's defined.
+  var initializationSteps = {};
+}
+
 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() {
@@ skipping 45 matching lines @@
   }
 }
 
 if (navigator.getVRDisplays) {
   frameData = new VRFrameData();
   navigator.getVRDisplays().then( (displays) => {
     if (displays.length > 0) {
       vrDisplay = displays[0];
     }
   }).then( () => {
-    vrDisplayPromiseDone = true;
+    initializationSteps["getVRDisplays"] = true;
   });
 } else {
-  vrDisplayPromiseDone = true;
+  initializationSteps["getVRDisplays"] = true;
 }
 
 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();