mash: Migrate ScopedTargetRootWindow to //ash/common

ash/common/wm_shell.cc

Index: ash/common/wm_shell.cc
diff --git a/ash/common/wm_shell.cc b/ash/common/wm_shell.cc
index 5ad467936d6084b6acf71476fb8afad4a51c1659..33f55e4ff4acf4ab97a4b0443701bc1252e03d18 100644
--- a/ash/common/wm_shell.cc
+++ b/ash/common/wm_shell.cc
@@ -167,7 +167,7 @@ bool WmShell::IsSystemModalWindowOpen() {
 void WmShell::ShowAppList() {
   // Show the app list on the default display for new windows.
   int64_t display_id =
-      GetRootWindowForNewWindows()->GetDisplayNearestWindow().id();
+      root_window_for_new_windows_->GetDisplayNearestWindow().id();
   delegate_->GetAppListPresenter()->Show(display_id);
 }
 
@@ -178,7 +178,7 @@ void WmShell::DismissAppList() {
 void WmShell::ToggleAppList() {
   // Show the app list on the default display for new windows.
   int64_t display_id =
-      GetRootWindowForNewWindows()->GetDisplayNearestWindow().id();
+      root_window_for_new_windows_->GetDisplayNearestWindow().id();
   delegate_->GetAppListPresenter()->ToggleAppList(display_id);
 }