Backport some tests in /ui/base/ime 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
/ui/base/ime.

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

This CL was uploaded by git cl split.

R=yhanada@chromium.org

Bug: 891670
Change-Id: I33f045fb6999ffc600ca04255547512bd81861d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786748
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694735}

ui/base/ime/input_method_base_unittest.cc

@@ -111,8 +111,8 @@ class ClientChangeVerifier {
 
 class InputMethodBaseTest : public testing::Test {
  private:
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::MainThreadType::UI};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::UI};
 };
 
 class MockInputMethodBase : public InputMethodBase {