Moves maintaining root_window_for_new_windows_ to Shell

ash/common/system/toast/toast_overlay.cc

Index: ash/common/system/toast/toast_overlay.cc
diff --git a/ash/common/system/toast/toast_overlay.cc b/ash/common/system/toast/toast_overlay.cc
index 5bebb73a8ee8bc6d3ba9c9690ffd68878fee1ab8..52d505112f052989bed8df6286a7de6b0612cfc5 100644
--- a/ash/common/system/toast/toast_overlay.cc
+++ b/ash/common/system/toast/toast_overlay.cc
@@ -5,10 +5,10 @@
 #include "ash/common/system/toast/toast_overlay.h"
 
 #include "ash/common/shelf/wm_shelf.h"
-#include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
 #include "ash/public/cpp/shell_window_ids.h"
 #include "ash/root_window_controller.h"
+#include "ash/shell.h"
 #include "ash/strings/grit/ash_strings.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
@@ -54,7 +54,7 @@ const int kToastMinimumWidth = 288;
 // Returns the work area bounds for the root window where new windows are added
 // (including new toasts).
 gfx::Rect GetUserWorkAreaBounds() {
-  return WmShelf::ForWindow(WmShell::Get()->GetRootWindowForNewWindows())
+  return WmShelf::ForWindow(Shell::GetInstance()->GetRootWindowForNewWindows())
       ->GetUserWorkAreaBounds();
 }
 
@@ -226,7 +226,7 @@ ToastOverlay::ToastOverlay(Delegate* delegate,
   params.remove_standard_frame = true;
   params.bounds = CalculateOverlayBounds();
   // Show toasts above the app list and below the lock screen.
-  WmShell::Get()
+  Shell::GetInstance()
       ->GetRootWindowForNewWindows()
       ->GetRootWindowController()
       ->ConfigureWidgetInitParamsForContainer(