Backport some tests in /components/discardable_memory 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/discardable_memory.

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

This CL was uploaded by git cl split.

R=penghuang@chromium.org

Bug: 891670
Change-Id: I968828bdca2039cf3a1cd3a752cdcd69c36110c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786948
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695134}

components/discardable_memory/service/discardable_shared_memory_manager_unittest.cc

@@ -66,7 +66,7 @@ class DiscardableSharedMemoryManagerTest : public testing::Test {
   }
 
   // DiscardableSharedMemoryManager requires a message loop.
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<TestDiscardableSharedMemoryManager> manager_;
 };
 
@@ -240,7 +240,7 @@ class DiscardableSharedMemoryManagerScheduleEnforceMemoryPolicyTest
   void SetUp() override { manager_.reset(new DiscardableSharedMemoryManager); }
 
   // DiscardableSharedMemoryManager requires a message loop.
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<DiscardableSharedMemoryManager> manager_;
 };