Make UserActivityDetector a singleton

ash/test/ash_test_helper.cc

Index: ash/test/ash_test_helper.cc
diff --git a/ash/test/ash_test_helper.cc b/ash/test/ash_test_helper.cc
index 86a50d5399736d6253fc5f22321da35f9ced4e27..2b746db4cea3b4143721978a239ea74a48cd5701 100644
--- a/ash/test/ash_test_helper.cc
+++ b/ash/test/ash_test_helper.cc
@@ -25,6 +25,7 @@
 #include "ui/compositor/test/context_factories_for_test.h"
 #include "ui/message_center/message_center.h"
 #include "ui/wm/core/capture_controller.h"
+#include "ui/wm/core/user_activity_detector.h"
 
 #if defined(OS_CHROMEOS)
 #include "chromeos/audio/cras_audio_handler.h"
@@ -87,6 +88,9 @@ void AshTestHelper::SetUp(bool start_session) {
   // Create CrasAudioHandler for testing since g_browser_process is not
   // created in AshTestBase tests.
   chromeos::CrasAudioHandler::InitializeForTesting();
+
+  // The UserActivityDetector must be created before the shell is created.
+  wm::UserActivityDetector::Create();
 #endif
   ShellInitParams init_params;
   init_params.delegate = test_shell_delegate_;
@@ -121,6 +125,8 @@ void AshTestHelper::TearDown() {
   message_center::MessageCenter::Shutdown();
 
 #if defined(OS_CHROMEOS)
+  wm::UserActivityDetector::Shutdown();
+
   chromeos::CrasAudioHandler::Shutdown();
   if (dbus_thread_manager_initialized_) {
     chromeos::DBusThreadManager::Shutdown();