Introduce window container to be used by lock screen app windows

ash/public/cpp/shell_window_ids.h

Index: ash/public/cpp/shell_window_ids.h
diff --git a/ash/public/cpp/shell_window_ids.h b/ash/public/cpp/shell_window_ids.h
index ab5a16840647c1a37abb98576acd4dedbbfc1aa4..d5ae3c0b2debe4fdcbec87c35b62336181906c08 100644
--- a/ash/public/cpp/shell_window_ids.h
+++ b/ash/public/cpp/shell_window_ids.h
@@ -77,6 +77,14 @@ enum ShellWindowId {
   // The container for the lock screen.
   kShellWindowId_LockScreenContainer,
 
+  // The container for windows that handle lock tray actions (e.g. new note
+  // action). The action handler container's children should be visible on lock
+  // screen, but only when an action is being handled - i.e. action handling
+  // state is either:
+  //  *  active - the container is stacked above LockScreenContainer
+  //  *  background - the container is stacked below LockScreenContainer
+  kShellWindowId_LockActionHandlerContainer,
+
   // The container for the lock screen modal windows.
   kShellWindowId_LockSystemModalContainer,
 
@@ -136,6 +144,7 @@ const int32_t kAllShellContainerIds[] = {
     kShellWindowId_SystemModalContainer,
     kShellWindowId_LockScreenWallpaperContainer,
     kShellWindowId_LockScreenContainer,
+    kShellWindowId_LockActionHandlerContainer,
     kShellWindowId_LockSystemModalContainer,
     kShellWindowId_StatusContainer,
     kShellWindowId_ImeWindowParentContainer,