Backport some tests in /content/browser/fileapi 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/fileapi.

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

This CL was uploaded by git cl split.

R=mek@chromium.org

Bug: 891670
Change-Id: Id40fcab691f3126d75b96599b381d622f6c6cc11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786814
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693848}

content/browser/fileapi/file_system_operation_runner_unittest.cc

@@ -87,7 +87,7 @@ class FileSystemOperationRunnerTest : public testing::Test {
 
  private:
   base::ScopedTempDir base_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   scoped_refptr<FileSystemContext> file_system_context_;
 
   DISALLOW_COPY_AND_ASSIGN(FileSystemOperationRunnerTest);