mash: Migrate ScopedTargetRootWindow to //ash/common

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 465e12e64a21ed7899ef51509e06e993a2a2b956..a5377574cbd0f1eb1fbb7743196797af3bb70b77 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -111,7 +111,6 @@ class ResizeShadowController;
 class ResolutionNotificationController;
 class RootWindowController;
 class ScopedOverviewAnimationSettingsFactoryAura;
-class ScopedTargetRootWindow;
 class ScreenAsh;
 class ScreenOrientationController;
 class ScreenshotController;
@@ -186,9 +185,10 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
 
   // Returns a root Window when used as a target when creating a new window.
   // The root window of the active window is used in most cases, but can
-  // be overridden by using ScopedTargetRootWindow().
+  // be overridden by using ScopedRootWindowForNewWindows.
   // If you want to get the root Window of the active window, just use
   // |wm::GetActiveWindow()->GetRootWindow()|.
+  // TODO(jamescook): Rename to GetRootWindowForNewWindows.
   static aura::Window* GetTargetRootWindow();
 
   // Returns the id of the display::Display corresponding to the window returned
@@ -480,7 +480,7 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   FRIEND_TEST_ALL_PREFIXES(WindowManagerTest, MouseEventCursors);
   FRIEND_TEST_ALL_PREFIXES(WindowManagerTest, TransformActivate);
   friend class RootWindowController;
-  friend class ScopedTargetRootWindow;
+  friend class ScopedRootWindowForNewWindowsAura;
   friend class test::ShellTestApi;
   friend class shell::WindowWatcher;