Fix flakes in ImageWriterWriteFromUrlOperationTest.VerifyFile

USER_VISIBLE tasks on the UI thread are now run at a lower priority
than default / USER_BLOCKING tasks.  This caused the test to become
flaky because it runs 100 USER_VISIBLE tasks to call OnProgress
but the QuitClocure task is run at USER_BLOCKING priority so
occasionally it can run too soon.  This patch fixes that by running
the Quit at USER_VISIBLE priority.

Change-Id: I93bc93c9c583b160aa1ff4326a021c432eab525a
BUG: 863341
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697204
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676874}

chrome/browser/extensions/api/image_writer_private/write_from_url_operation_unittest.cc

@@ -238,7 +238,18 @@ TEST_F(ImageWriterWriteFromUrlOperationTest, DISABLED_VerifyFile) {
   operation->SetImagePath(test_utils_.GetImagePath());
   {
     base::RunLoop run_loop;
-    operation->VerifyDownload(run_loop.QuitClosure());
+    // The OnProgress tasks are posted with priority USER_VISIBLE priority so
+    // post the quit closure with the same priority to ensure it doesn't run too
+    // soon.
+    operation->VerifyDownload(base::Bind(
+        [](base::OnceClosure quit_closure) {
+          base::PostTaskWithTraits(
+              FROM_HERE,
+              {content::BrowserThread::UI, base::TaskPriority::USER_VISIBLE},
+              std::move(quit_closure));
+        },
+        run_loop.QuitClosure()));
+
     run_loop.Run();
   }