[FileAPI] Fix flaky FileTest.*.

WTF::String (and the underlying StringImpl) is tied to a particular
platform thread as opposed to a sequence. As such it is not safe to own
one on a SequencedTaskRunner. So change to using a SingleThreadTaskRunner
for background work instead.

Bug: 1038444
Change-Id: If4391d476ab181a7adeefcf734d96a1802a28626
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986210Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728096}

third_party/blink/renderer/core/fileapi/file_test.cc

@@ -26,7 +26,7 @@ class MockBlob : public FakeBlob {
   static void Create(File* file, base::Time modified_time) {
     mojo::PendingRemote<mojom::blink::Blob> remote;
     PostCrossThreadTask(
-        *base::CreateSequencedTaskRunner({base::ThreadPool()}), FROM_HERE,
+        *base::CreateSingleThreadTaskRunner({base::ThreadPool()}), FROM_HERE,
         CrossThreadBindOnce(
             [](const String& uuid,
                mojo::PendingReceiver<mojom::blink::Blob> receiver,