Use ScopedTaskEnvironment instead of MessageLoopForUI in ash tests.

ScopedTaskEnvironment allows usage of ThreadTaskRunnerHandle and
base/task_scheduler/post_task.h within its scope. It should be
instantiated in everytest that uses either of these APIs
(i.e. no test should instantiate a MessageLoop directly).

Motivation for ScopedTaskEnvironment can be found in:
https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit

BUG=708584
R=gab@chromium.org
TBR=sky@chromium.org

Review-Url: https://codereview.chromium.org/2846723004
Cr-Commit-Position: refs/heads/master@{#467987}

ash/system/power/power_status_unittest.cc

@@ -7,8 +7,8 @@
 #include <memory>
 
 #include "ash/resources/vector_icons/vector_icons.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "chromeos/dbus/dbus_thread_manager.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/cros_system_api/dbus/service_constants.h"
@@ -38,7 +38,10 @@ class TestObserver : public PowerStatus::Observer {
 
 class PowerStatusTest : public testing::Test {
  public:
-  PowerStatusTest() : power_status_(NULL) {}
+  PowerStatusTest()
+      : scoped_task_environment_(
+            base::test::ScopedTaskEnvironment::MainThreadType::UI),
+        power_status_(NULL) {}
   ~PowerStatusTest() override {}
 
   void SetUp() override {
@@ -57,7 +60,7 @@ class PowerStatusTest : public testing::Test {
   }
 
  protected:
-  base::MessageLoopForUI message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   PowerStatus* power_status_;  // Not owned.
   std::unique_ptr<TestObserver> test_observer_;
 

ash/test/ash_test_environment_default.cc

@@ -6,8 +6,8 @@
 
 #include "ash/test/ash_test_views_delegate.h"
 #include "base/memory/ptr_util.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/test/sequenced_worker_pool_owner.h"
 #include "base/threading/sequenced_worker_pool.h"
 
@@ -17,7 +17,9 @@ namespace {
 
 class AshTestEnvironmentDefault : public AshTestEnvironment {
  public:
-  AshTestEnvironmentDefault() {}
+  AshTestEnvironmentDefault()
+      : scoped_task_environment_(
+            base::test::ScopedTaskEnvironment::MainThreadType::UI) {}
 
   ~AshTestEnvironmentDefault() override {
     base::RunLoop().RunUntilIdle();
@@ -40,7 +42,7 @@ class AshTestEnvironmentDefault : public AshTestEnvironment {
   }
 
  private:
-  base::MessageLoopForUI message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   std::unique_ptr<base::SequencedWorkerPoolOwner> blocking_pool_owner_;
 
   DISALLOW_COPY_AND_ASSIGN(AshTestEnvironmentDefault);