Eliminate ash::Shell access in ShelfView

ash/aura/wm_shell_aura.cc

Index: ash/aura/wm_shell_aura.cc
diff --git a/ash/aura/wm_shell_aura.cc b/ash/aura/wm_shell_aura.cc
index 3908dad13ef87617561c0a48093cb7b995979f1a..29af4b96cb5e2fec943568fa8f1b110694aab52f 100644
--- a/ash/aura/wm_shell_aura.cc
+++ b/ash/aura/wm_shell_aura.cc
@@ -141,6 +141,11 @@ void WmShellAura::RecordTaskSwitchMetric(TaskSwitchSource source) {
       source);
 }
 
+void WmShellAura::ShowContextMenu(const gfx::Point& location_in_screen,
+                                  ui::MenuSourceType source_type) {
+  Shell::GetInstance()->ShowContextMenu(location_in_screen, source_type);
+}
+
 std::unique_ptr<WindowResizer> WmShellAura::CreateDragWindowResizer(
     std::unique_ptr<WindowResizer> next_window_resizer,
     wm::WindowState* window_state) {