[generic-sensor] Avoid unhandled promise rejection (#16414)

By testing for a global binding via an unqualified IdentifierReference,
the Generic Sensor utility file produces a rejected promise which is not
handled. If testharness.js has not yet determined that testing is
complete, this unhandled rejection will cause a harness-level error.
Such errors will occur based on relative timing of asynchronous
operations, and this has been observed to be unstable in Apple Safari as
hosted on Azure Pipelines.

Update the feature detection logic to avoid generating an unhandled
rejection by instead referencing a property of the global object. Update
the subsquent assertion to provide a more clear explanation of the
failure which occurs in non-supporting browsers.

generic-sensor/generic-sensor-tests.js

@@ -7,7 +7,7 @@
 //
 //   --enable-blink-features=MojoJS,MojoJSTest
 let loadChromiumResources = Promise.resolve().then(() => {
-  if (!MojoInterfaceInterceptor) {
+  if (!window.MojoInterfaceInterceptor) {
     // Do nothing on non-Chromium-based browsers or when the Mojo bindings are
     // not present in the global namespace.
     return;
@@ -37,7 +37,10 @@ async function initialize_generic_sensor_tests() {
   if (typeof GenericSensorTest === 'undefined') {
     await loadChromiumResources;
   }
-  assert_true(typeof GenericSensorTest !== 'undefined');
+  assert_true(
+    typeof GenericSensorTest !== 'undefined',
+    'Mojo testing interface is not available.'
+  );
   let sensorTest = new GenericSensorTest();
   await sensorTest.initialize();
   return sensorTest;