mash: Port context menu code to WmShell and WmRootWindowController.

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index 9d34fe16a1914aab80f245d67c37d90406946776..456528afc1888848a738b0445aa88f2ef63638a0 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -318,12 +318,6 @@ void WmShellMus::RecordTaskSwitchMetric(TaskSwitchSource source) {
   NOTIMPLEMENTED();
 }
 
-void WmShellMus::ShowContextMenu(const gfx::Point& location_in_screen,
-                                 ui::MenuSourceType source_type) {
-  // TODO: http://crbug.com/640693.
-  NOTIMPLEMENTED();
-}
-
 std::unique_ptr<WindowResizer> WmShellMus::CreateDragWindowResizer(
     std::unique_ptr<WindowResizer> next_window_resizer,
     wm::WindowState* window_state) {