Better generalize VR test framework

chrome/test/data/android/webvr_instrumentation/resources/webvr_e2e.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 testPassed = false;
 var resultString = "";
-var asyncCounter = 0;
 var javascriptDone = false;
-var vrDisplayPromiseDone = false;
+var initializationSteps = {load: false};
 
 function finishJavaScriptStep() {
   javascriptDone = true;
 }
 
+// Used to check when JavaScript is in an acceptable state to start testing
+// after a page load, as Chrome thinking that the page has finished loading
+// is not always sufficient. By default waits until the load event is fired.
+function isInitializationComplete() {
+  for (var step in initializationSteps) {
+    if (!initializationSteps[step]) {
+      return false;
+    }
+  }
+  return true;
+}
+
+window.addEventListener("load",
+    () => {initializationSteps["load"] = true;}, false);
+
 function checkResultsForFailures(tests, harness_status) {
   testPassed = true;
   if (harness_status["status"] != 0) {
     testPassed = false;
     resultString += "Harness failed due to " +
                     (harness_status["status"] == 1 ? "error" : "timeout")
                     + ". ";
   }
   for (var test in tests) {
     let passed = (tests[test]["status"] == 0);
     if (!passed) {
       testPassed = false;
       resultString += "FAIL ";
     } else {
       resultString += "PASS ";
     }
     resultString += tests[test]["name"] +
                     (passed ? "" : (": " + tests[test]["message"])) +
                     ". ";
   }
 }
 
-add_completion_callback( (tests, harness_status) => {
-  checkResultsForFailures(tests, harness_status);
-  console.debug("Test result: " + (testPassed ? "Pass" : "Fail"));
-  console.debug("Test result string: " + resultString);
-  finishJavaScriptStep();
-});
+// Only interact with testharness.js if it was actually included on the page
+// before this file
+if (typeof add_completion_callback !== "undefined") {
+  add_completion_callback( (tests, harness_status) => {
+    checkResultsForFailures(tests, harness_status);
+    console.debug("Test result: " + (testPassed ? "Pass" : "Fail"));
+    console.debug("Test result string: " + resultString);
+    finishJavaScriptStep();
+  });
+}