Backport some tests in...

Backport some tests in /components/autofill/core/browser/webdata/autocomplete_sync_bridge_unittest.cc 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
/components/autofill/core/browser/webdata/autocomplete_sync_bridge_unittest.cc.

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

This CL was uploaded by git cl split.

R=melandory@chromium.org

Bug: 891670
Change-Id: I46ee96d64d85db8fc4f320ef5a601d5bb7f42b88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786806
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Tatiana Gornak <melandory@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695131}

components/autofill/core/browser/webdata/autocomplete_sync_bridge_unittest.cc

@@ -293,7 +293,7 @@ class AutocompleteSyncBridgeTest : public testing::Test {
 
  private:
   ScopedTempDir temp_dir_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   testing::NiceMock<MockAutofillWebDataBackend> backend_;
   AutofillTable table_;
   WebDatabase db_;