Fix "modal isn't modal in multi displays" issue

ash/wm/system_modal_container_layout_manager.cc

Index: ash/wm/system_modal_container_layout_manager.cc
diff --git a/ash/wm/system_modal_container_layout_manager.cc b/ash/wm/system_modal_container_layout_manager.cc
index 8a9e859fcc8f14e3904415e4fc7988e384613f12..83e7e128e5c6dc72e75f725f225e697e58e0b700 100644
--- a/ash/wm/system_modal_container_layout_manager.cc
+++ b/ash/wm/system_modal_container_layout_manager.cc
@@ -113,22 +113,10 @@ void SystemModalContainerLayoutManager::OnKeyboardBoundsChanging(
   PositionDialogsAfterWorkAreaResize();
 }
 
-bool SystemModalContainerLayoutManager::CanWindowReceiveEvents(
+bool SystemModalContainerLayoutManager::IsActiveModalWindows(
     aura::Window* window) {
-  // We could get when we're at lock screen and there is modal window at
-  // system modal window layer which added event filter.
-  // Now this lock modal windows layer layout manager should not block events
-  // for windows at lock layer.
-  // See SystemModalContainerLayoutManagerTest.EventFocusContainers and
-  // http://crbug.com/157469
-  if (modal_windows_.empty())
-    return true;
-  // This container can not handle events if the screen is locked and it is not
-  // above the lock screen layer (crbug.com/110920).
-  if (Shell::GetInstance()->session_state_delegate()->IsUserSessionBlocked() &&
-      container_->id() < ash::kShellWindowId_LockScreenContainer)
-    return true;
-  return wm::GetActivatableWindow(window) == modal_window();
+  return modal_window() != nullptr &&

[James Cook, 2016/06/16 20:31:35]

For my knowledge: Do we now prefer comparing to nullptr vs. if (!pointer) and if
(pointer) ?

[oshima, 2016/06/17 00:06:12]

I saw the discussion but I haven't read to the conslusion. I'm happy to change,
so let me know

No, I don't think there is preference. Changed to !pointer to be consistent. (I
think UI code tends to prefer this, probably because it's shorter)

I've seen a recent discussion but I don't think there was any consensus on this.

[James Cook, 2016/06/17 16:04:43]

I prefer !pointer myself, just for consistency with old code.

(Out of curiosity, on which mailing list have you seen these discussions?
Chromium-dev?)

+         wm::GetActivatableWindow(window) == modal_window();
 }
 
 bool SystemModalContainerLayoutManager::ActivateNextModalWindow() {