WPT: Refactor screen_enumeration tests to use test-only-api.js

use of test-only-api.js in preparation for launching official MojoJS support
in WPT. This would not change the test results on Chromium waterfall
(everything should continue to pass) or upstream WPT (tests currently
fail because MojoJS isn't enabled).

Bug: 1123992
Change-Id: I513e6d432f6c74a62109ea09ce0dbaeefc63b0c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391224
Commit-Queue: Arnaud Mandy <arnaud.mandy@intel.com>
Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805064}

third_party/blink/web_tests/external/wpt/screen_enumeration/resources/screenenumeration-helpers.js

@@ -6,45 +6,40 @@
 //
 //   --enable-blink-features=MojoJS,MojoJSTest
 
-const loadChromiumResources = async () => {
-  if (!("MojoInterfaceInterceptor" in self)) {
-    // Do nothing on non-Chromium-based browsers or when the Mojo bindings are
-    // not present in the global namespace.
-    return;
-  }
-
-  const resources = [
-    "/gen/layout_test_data/mojo/public/js/mojo_bindings.js",
-    "/gen/ui/gfx/mojom/color_space.mojom.js",
-    "/gen/ui/gfx/mojom/buffer_types.mojom.js",
-    "/gen/ui/gfx/mojom/display_color_spaces.mojom.js",
-    "/gen/ui/gfx/geometry/mojom/geometry.mojom.js",
-    "/gen/ui/display/mojom/display.mojom.js",
-    "/gen/third_party/blink/public/mojom/screen_enumeration/screen_enumeration.mojom.js",
-    "/resources/testdriver.js",
-    "/resources/testdriver-vendor.js",
-    "/resources/chromium/mock-screenenumeration.js",
+async function loadChromiumResources() {
+  const chromiumResources = [
+    '/gen/ui/gfx/mojom/color_space.mojom.js',
+    '/gen/ui/gfx/mojom/buffer_types.mojom.js',
+    '/gen/ui/gfx/mojom/display_color_spaces.mojom.js',
+    '/gen/ui/gfx/geometry/mojom/geometry.mojom.js',
+    '/gen/ui/display/mojom/display.mojom.js',
+    '/gen/third_party/blink/public/mojom/screen_enumeration/screen_enumeration.mojom.js'
   ];
-  await Promise.all(resources.map(path => {
-    const script = document.createElement("script");
-    script.src = path;
-    script.async = false;
-    const promise = new Promise((resolve, reject) => {
-      script.onload = resolve;
-      script.onerror = reject;
-    });
-    document.head.appendChild(script);
-    return promise;
-  }));
 
-};
+  await loadMojoResources(chromiumResources);
+  await loadScript('/resources/testdriver.js');
+  await loadScript('/resources/testdriver-vendor.js');
+  await loadScript('/resources/chromium/mock-screenenumeration.js');
+}
 
 async function initialize_screen_enumeration_tests() {
-  if (typeof ScreenEnumerationTest === "undefined")
-    await loadChromiumResources();
+  if (typeof ScreenEnumerationTest === "undefined") {
+    const script = document.createElement('script');
+    script.src = '/resources/test-only-api.js';
+    script.async = false;
+    const p = new Promise((resolve, reject) => {
+      script.onload = () => { resolve(); };
+      script.onerror = e => { reject(e); };
+    });
+    document.head.appendChild(script);
+    await p;
 
-  assert_true(typeof ScreenEnumerationTest !== "undefined",
-              "Screen Enumeration testing interface is not available.");
+    if (isChromiumBased) {
+      await loadChromiumResources();
+    }
+  }
+  assert_implements(ScreenEnumerationTest,
+                    'Screen Enumeration testing interface is not available.');
   let enumTest = new ScreenEnumerationTest();
   await enumTest.initialize();
   return enumTest;