mash: Remove last WmWindow uses in ash/shelf.

ash/shelf/overflow_bubble_view.cc

Index: ash/shelf/overflow_bubble_view.cc
diff --git a/ash/shelf/overflow_bubble_view.cc b/ash/shelf/overflow_bubble_view.cc
index 22af1f058a92845753fc14d34bcf70f804c960e3..de12a4920100e72e5b657d5993d23ceb8ed0e3d1 100644
--- a/ash/shelf/overflow_bubble_view.cc
+++ b/ash/shelf/overflow_bubble_view.cc
@@ -11,7 +11,6 @@
 #include "ash/shelf/shelf.h"
 #include "ash/shelf/shelf_constants.h"
 #include "ash/shell.h"
-#include "ash/wm_window.h"
 #include "base/i18n/rtl.h"
 #include "ui/display/display.h"
 #include "ui/display/screen.h"
@@ -163,8 +162,7 @@ void OverflowBubbleView::OnBeforeBubbleWidgetInit(
     views::Widget::InitParams* params,
     views::Widget* bubble_widget) const {
   // Place the bubble in the same root window as the anchor.
-  WmWindow::Get(anchor_widget()->GetNativeWindow())
-      ->GetRootWindowController()
+  RootWindowController::ForWindow(anchor_widget()->GetNativeWindow())
       ->ConfigureWidgetInitParamsForContainer(
           bubble_widget, kShellWindowId_ShelfBubbleContainer, params);
 }