Backport some tests in /chrome/browser/ui/omnibox 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
/chrome/browser/ui/omnibox.

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

This CL was uploaded by git cl split.

R=tommycli@chromium.org

Bug: 891670
Change-Id: I1b56026765dadc3aa29b28eed731e7b09f8541c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786803
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694573}

chrome/browser/ui/omnibox/clipboard_utils_unittest.cc

@@ -24,7 +24,8 @@ namespace {
 class ClipboardUtilsTest : public PlatformTest {
  public:
   ClipboardUtilsTest()
-      : task_environment_(base::test::TaskEnvironment::MainThreadType::UI) {}
+      : task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::UI) {}
 
   void SetUp() override {
     PlatformTest::SetUp();
@@ -38,7 +39,7 @@ class ClipboardUtilsTest : public PlatformTest {
 
  private:
   // Windows requires a message loop for clipboard access.
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 TEST_F(ClipboardUtilsTest, GetClipboardText) {