cros: Fix flaky TouchAccessibilityEnablerTest

TouchAccessibilityEnablerTest.InteractsWithTouchExplorationController
is flaky in ash_unittests because ScreenForShutdown persists between
tests. Clean up ScreenForShutdown at the end of each AshTestBase.

Bug: 844253
Test: ash_unittests
Change-Id: I4437fea28bc7757dc38d27db8bfbd173ddfba07a
Reviewed-on: https://chromium-review.googlesource.com/1065177Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560230}

ash/display/screen_ash.cc

@@ -27,8 +27,7 @@ namespace ash {
 
 namespace {
 
-// We need to keep this in order for unittests to tell if
-// the object in display::Screen::GetScreenByType is for shutdown.
+// Intentionally leaked in production.
 display::Screen* screen_for_shutdown = nullptr;
 
 display::DisplayManager* GetDisplayManager() {
@@ -202,4 +201,12 @@ void ScreenAsh::CreateScreenForShutdown() {
   display::Screen::SetScreenInstance(screen_for_shutdown);
 }
 
+// static
+void ScreenAsh::DeleteScreenForShutdown() {
+  if (display::Screen::GetScreen() == screen_for_shutdown)
+    display::Screen::SetScreenInstance(nullptr);
+  delete screen_for_shutdown;
+  screen_for_shutdown = nullptr;
+}
+
 }  // namespace ash

ash/display/screen_ash.h

@@ -51,6 +51,9 @@ class ASH_EXPORT ScreenAsh : public display::Screen {
   // Create a screen instance to be used during shutdown.
   static void CreateScreenForShutdown();
 
+  // Test helpers may need to clean up the ScreenForShutdown between tests.
+  static void DeleteScreenForShutdown();
+
  private:
   DISALLOW_COPY_AND_ASSIGN(ScreenAsh);
 };

ash/test/ash_test_base.cc

@@ -12,6 +12,7 @@
 #include "ash/app_list/test/app_list_test_helper.h"
 #include "ash/display/extended_mouse_warp_controller.h"
 #include "ash/display/mouse_cursor_event_filter.h"
+#include "ash/display/screen_ash.h"
 #include "ash/display/screen_orientation_controller_test_api.h"
 #include "ash/display/unified_mouse_warp_controller.h"
 #include "ash/display/window_tree_host_manager.h"
@@ -146,6 +147,10 @@ AshTestBase::~AshTestBase() {
       << "You have overridden TearDown but never called AshTestBase::TearDown";
   if (AshTestHelper::config() != Config::CLASSIC)
     aura::Env::GetInstance()->RemoveObserver(this);
+
+  // Ensure the next test starts with a null display::Screen. Done here because
+  // some tests use Screen in their TearDown() overrides.
+  ScreenAsh::DeleteScreenForShutdown();
 }
 
 void AshTestBase::SetUp() {