Better generalize VR test framework

chrome/test/data/android/webvr_instrumentation/resources/webvr_e2e.js

Index: chrome/test/data/android/webvr_instrumentation/resources/webvr_e2e.js
diff --git a/chrome/test/data/android/webvr_instrumentation/resources/webvr_e2e.js b/chrome/test/data/android/webvr_instrumentation/resources/webvr_e2e.js
index 03f006cb53132eeb496354a78c88589773bbcf45..abf05cb4af39fe5c6adcaacb8e332950c100b672 100644
--- a/chrome/test/data/android/webvr_instrumentation/resources/webvr_e2e.js
+++ b/chrome/test/data/android/webvr_instrumentation/resources/webvr_e2e.js
@@ -4,14 +4,28 @@
 
 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) {
@@ -34,9 +48,13 @@ function checkResultsForFailures(tests, harness_status) {
   }
 }
 
-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();
+  });
+}