Clean up some chrome://media-app/../drop_target.js remnants.

This used to be loaded at runtime. But now it is not.

Bug: b/162187144
Change-Id: Ia627641964149ae754c2d5b086fd8c261788be64
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437559Reviewed-by: dstockwell <dstockwell@google.com>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812002}

chromeos/components/media_app_ui/media_app_guest_ui.cc

@@ -33,8 +33,6 @@ content::WebUIDataSource* CreateMediaAppUntrustedDataSource(
   source->AddResourcePath("js/app_main.js", IDR_MEDIA_APP_APP_MAIN_JS);
   source->AddResourcePath("js/app_image_handler_module.js",
                           IDR_MEDIA_APP_APP_IMAGE_HANDLER_MODULE_JS);
-  source->AddResourcePath("js/app_drop_target_module.js",
-                          IDR_MEDIA_APP_APP_DROP_TARGET_MODULE_JS);
 
   // Add all resources from chromeos_media_app_bundle_resources.pak.
   for (size_t i = 0; i < kChromeosMediaAppBundleResourcesSize; i++) {

chromeos/components/media_app_ui/resources/mock/media_app_bundle_mock_resources.grd

@@ -18,10 +18,6 @@
           file="js/mock_module.js"
           compress="brotli"
           type="BINDATA" />
-      <include name="IDR_MEDIA_APP_APP_DROP_TARGET_MODULE_JS"
-          file="js/mock_module.js"
-          compress="brotli"
-          type="BINDATA" />
       <!-- Use a placeholder icon when enable_cros_media_app = false. -->
       <include name="IDR_MEDIA_APP_APP_ICON_256_PNG"
             file="../../../../../ui/file_manager/gallery/images/icon256.png"

chromeos/components/media_app_ui/test/media_app_guest_ui_browsertest.js

@@ -6,16 +6,34 @@
 
 // Test web workers can be spawned from chrome-untrusted://media-app. Errors
 // will be logged in console from web_ui_browser_test.cc.
-GUEST_TEST('GuestCanSpawnWorkers', () => {
-  let error = null;
+GUEST_TEST('GuestCanSpawnWorkers', async () => {
+  let caughtError = null;
+  let networkErrorEventType = '';
 
   try {
-    const worker = new Worker('js/app_drop_target_module.js');
+    // The "real" webworker isn't needed for the mock, but we want to test CSP
+    // errors. Fetch something that doesn't exist. This has a bonus that we can
+    // wait for an "error" event, whereas fetching a "real" file would require
+    // it to respond to a postMessage. Note the resource will 404, but that
+    // won't throw an exception. CSP errors, however, will throw errors that
+    // fail the test. E.g., Failed to construct 'Worker': Script at
+    // 'about:blank' cannot be accessed from origin
+    // 'chrome-untrusted://media-app'.
+    const worker = new Worker('/non-existent.js');
+
+    // Await the network error to ensure things get that far. The error itself
+    // is entirely opaque.
+    networkErrorEventType = await new Promise(resolve => {
+      worker.onerror = (event) => {
+        resolve(event.type);
+      };
+    });
   } catch (e) {
-    error = e;
+    caughtError = e;
   }
 
-  assertEquals(error, null, error && error.message);
+  assertEquals(caughtError, null, caughtError && caughtError.message);
+  assertEquals(networkErrorEventType, 'error');
 });
 
 // Test that language is set correctly on the guest iframe.