mash: Port context menu code to WmShell and WmRootWindowController.

ash/common/wm_shell.h

Index: ash/common/wm_shell.h
diff --git a/ash/common/wm_shell.h b/ash/common/wm_shell.h
index 00ac9ada5f2b5bd9f7386dd12d2f3a424533005f..ac6927a0b1d1b72b66c753b0dea13b5bad38b6f9 100644
--- a/ash/common/wm_shell.h
+++ b/ash/common/wm_shell.h
@@ -277,10 +277,9 @@ class ASH_EXPORT WmShell {
   virtual void RecordUserMetricsAction(UserMetricsAction action) = 0;
   virtual void RecordTaskSwitchMetric(TaskSwitchSource source) = 0;
 
-  // Shows the context menu for the background and the shelf at
-  // |location_in_screen|.
-  virtual void ShowContextMenu(const gfx::Point& location_in_screen,
-                               ui::MenuSourceType source_type) = 0;
+  // Shows the context menu for the wallpaper or shelf at |location_in_screen|.
+  void ShowContextMenu(const gfx::Point& location_in_screen,
+                       ui::MenuSourceType source_type);
 
   // Returns a WindowResizer to handle dragging. |next_window_resizer| is
   // the next WindowResizer in the WindowResizer chain. This may return