[WPT] Use assert_implements better in webusb, webxr

With https://github.com/web-platform-tests/rfcs/pull/28, we can now use
assert_* freely in any (including the first) test function without
worrying about turning the test into a single-page test accidentally.
This CL takes advantage of that and makes better use of
assert_implements to convey failure messages.

Change-Id: I328fdbb31195b02e3f97e16cc9f1f1cacbe03f7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359163
Commit-Queue: Robert Ma <robertma@chromium.org>
Auto-Submit: Robert Ma <robertma@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799080}

third_party/blink/web_tests/external/wpt/webusb/resources/usb-helpers.js

@@ -33,15 +33,15 @@ async function loadChromiumResources() {
 
 function usb_test(func, name, properties) {
   promise_test(async () => {
+    assert_implements(navigator.usb, 'missing navigator.usb');
     if (navigator.usb.test === undefined) {
       // Try loading a polyfill for the WebUSB Testing API.
       if (isChromiumBased) {
         await loadChromiumResources();
         await loadScript('/resources/chromium/webusb-test.js');
-      } else {
-        assert_implements(false, "missing navigator.usb.test");
       }
     }
+    assert_implements(navigator.usb.test, 'missing navigator.usb.test after initialization');
 
     await navigator.usb.test.initialize();
     try {

third_party/blink/web_tests/external/wpt/webxr/resources/webxr_util.js

@@ -18,6 +18,8 @@ function xr_promise_test(name, func, properties) {
     // Perform any required test setup:
     xr_debug(name, 'setup');
 
+    assert_implements(navigator.xr, 'missing navigator.xr');
+
     // Only set up once.
     if (!navigator.xr.test) {
       // Load test-only API helpers.
@@ -37,8 +39,6 @@ function xr_promise_test(name, func, properties) {
       } else if (isWebKitBased) {
         // WebKit setup
         await setupWebKitWebXRTestAPI();
-      } else {
-        assert_implements(false, "missing navigator.xr.test");
       }
     }
 
@@ -47,11 +47,7 @@ function xr_promise_test(name, func, properties) {
     // setup properly. Either way, the fact that xr_promise_test is being used
     // means that the tests expect navigator.xr.test to be set. By rejecting now
     // we can hopefully provide a clearer indication of what went wrong.
-    if (!navigator.xr.test) {
-      // We can't use assert_true here because it causes the wpt testharness
-      // to treat this as a test page and not as a test.
-      return Promise.reject("No navigator.xr.test object found, even after attempted load");
-    }
+    assert_implements(navigator.xr.test, 'missing navigator.xr.test, even after attempted load');
 
     // Ensure that any devices are disconnected when done. If this were done in
     // a .then() for the success case, a test that expected failure would