[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in discardable_memory

MessageLoop will go away, eventually.

BUG=891670

Change-Id: I7b07813a9fe1a48bfe4f268d96c5cbe3fb0f8542
Reviewed-on: https://chromium-review.googlesource.com/c/1352777Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#612186}

components/discardable_memory/service/BUILD.gn

@@ -32,6 +32,7 @@ source_set("unit_tests") {
   deps = [
     ":service",
     "//base",
+    "//base/test:test_support",
     "//testing/gtest",
   ]
 }

components/discardable_memory/service/discardable_shared_memory_manager_unittest.cc

@@ -7,7 +7,7 @@
 #include <stdint.h>
 #include <string.h>
 
-#include "base/message_loop/message_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/simple_thread.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -66,7 +66,7 @@ class DiscardableSharedMemoryManagerTest : public testing::Test {
   }
 
   // DiscardableSharedMemoryManager requires a message loop.
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment 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::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment task_environment_;
   std::unique_ptr<DiscardableSharedMemoryManager> manager_;
 };