Backport some tests in...

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

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

This CL was uploaded by git cl split.

R=vitaliii@chromium.org

Bug: 891670
Change-Id: I2716a45c81b330e553938972f2d856dc96198ac3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786751
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: vitaliii <vitaliii@chromium.org>
Commit-Queue: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694222}

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

@@ -451,7 +451,7 @@ class AutofillWalletMetadataSyncBridgeTest : public testing::Test {
   int response_version = 0;
   autofill::TestAutofillClock test_clock_;
   ScopedTempDir temp_dir_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   testing::NiceMock<MockAutofillWebDataBackend> backend_;
   AutofillTable table_;
   WebDatabase db_;