[TaskEnvironment] Get rid of the last explicit usages of MAIN_THREAD_ONLY that...

[TaskEnvironment] Get rid of the last explicit usages of MAIN_THREAD_ONLY that snuck in during the transition

TBR=dcheng@chromium.org

Bug: 992483
Change-Id: I1c95dfd52690fdedb0ae35ffa7da9081e2af182f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796753Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695392}

android_webview/javatests/src/org/chromium/android_webview/test/memory_metrics_logger_test.cc

@@ -19,8 +19,7 @@ jboolean JNI_MemoryMetricsLoggerTest_ForceRecordHistograms(JNIEnv* env) {
     return false;
 
   TestTimeouts::Initialize();
-  base::test::TaskEnvironment task_environment(
-      base::test::TaskEnvironment::ThreadingMode::MAIN_THREAD_ONLY);
+  base::test::SingleThreadTaskEnvironment task_environment;
   base::RunLoop run_loop;
   bool result = false;
   memory_metrics_logger->ScheduleRecordForTesting(

chrome/browser/chromeos/policy/app_install_event_log_uploader_unittest.cc

@@ -82,11 +82,7 @@ class MockAppInstallEventLogUploaderDelegate
 
 class AppInstallEventLogUploaderTest : public testing::Test {
  protected:
-  AppInstallEventLogUploaderTest()
-      : task_environment_(
-            base::test::TaskEnvironment::TimeSource::MOCK_TIME,
-            base::test::TaskEnvironment::ThreadingMode::MAIN_THREAD_ONLY),
-        value_report_(base::Value::Type::DICTIONARY) {}
+  AppInstallEventLogUploaderTest() = default;
 
   void TearDown() override {
     Mock::VerifyAndClearExpectations(&client_);
@@ -163,9 +159,10 @@ class AppInstallEventLogUploaderTest : public testing::Test {
     CaptureUpload(callback);
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::TaskEnvironment::TimeSource::MOCK_TIME};
   em::AppInstallReportRequest log_;
-  base::Value value_report_;
+  base::Value value_report_{base::Value::Type::DICTIONARY};
 
   MockCloudPolicyClient client_;
   MockAppInstallEventLogUploaderDelegate delegate_;