Backport some tests in /content/browser to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/content/browser.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=boliu@chromium.org

Bug: 891670
Change-Id: I7950058b91b35637f160a179f66af7c3a8abff6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787208
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693696}

content/browser/byte_stream_unittest.cc

@@ -91,7 +91,7 @@ class ByteStreamTest : public testing::Test {
   }
 
  protected:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 
  private:
   int producing_seed_key_;

content/browser/shareable_file_reference_unittest.cc

@@ -17,7 +17,7 @@ using storage::ShareableFileReference;
 namespace content {
 
 TEST(ShareableFileReferenceTest, TestReferences) {
-  base::test::TaskEnvironment task_environment;
+  base::test::SingleThreadTaskEnvironment task_environment;
   scoped_refptr<base::SingleThreadTaskRunner> task_runner =
       base::ThreadTaskRunnerHandle::Get();
   base::ScopedTempDir temp_dir;