Backport some tests in /components/storage_monitor 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/storage_monitor.

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: I34456e867324f407da9ea91221db343c7daa6f84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787105
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@{#694574}

components/storage_monitor/storage_monitor_unittest.cc

@@ -24,7 +24,7 @@ void SetLatch(bool* called) {
 namespace storage_monitor {
 
 TEST(StorageMonitorTest, TestInitialize) {
-  base::test::TaskEnvironment task_environment;
+  base::test::SingleThreadTaskEnvironment task_environment;
   TestStorageMonitor::Destroy();
   TestStorageMonitor monitor;
   EXPECT_FALSE(monitor.init_called());
@@ -39,7 +39,7 @@ TEST(StorageMonitorTest, TestInitialize) {
 
 TEST(StorageMonitorTest, DeviceAttachDetachNotifications) {
   TestStorageMonitor::Destroy();
-  base::test::TaskEnvironment task_environment;
+  base::test::SingleThreadTaskEnvironment task_environment;
   const std::string kDeviceId1 = "dcim:UUID:FFF0-0001";
   const std::string kDeviceId2 = "dcim:UUID:FFF0-0002";
   MockRemovableStorageObserver observer1;
@@ -82,7 +82,7 @@ TEST(StorageMonitorTest, DeviceAttachDetachNotifications) {
 
 TEST(StorageMonitorTest, GetAllAvailableStoragesEmpty) {
   TestStorageMonitor::Destroy();
-  base::test::TaskEnvironment task_environment;
+  base::test::SingleThreadTaskEnvironment task_environment;
   TestStorageMonitor monitor;
   std::vector<StorageInfo> devices = monitor.GetAllAvailableStorages();
   EXPECT_EQ(0U, devices.size());
@@ -90,7 +90,7 @@ TEST(StorageMonitorTest, GetAllAvailableStoragesEmpty) {
 
 TEST(StorageMonitorTest, GetAllAvailableStorageAttachDetach) {
   TestStorageMonitor::Destroy();
-  base::test::TaskEnvironment task_environment;
+  base::test::SingleThreadTaskEnvironment task_environment;
   TestStorageMonitor monitor;
   const std::string kDeviceId1 = "dcim:UUID:FFF0-0042";
   const base::FilePath kDevicePath1(FILE_PATH_LITERAL("/testfoo"));