mus: Add PointerWatcher for passively observing mouse and touch events

ash/shell/shell_delegate_impl.h

Index: ash/shell/shell_delegate_impl.h
diff --git a/ash/shell/shell_delegate_impl.h b/ash/shell/shell_delegate_impl.h
index daead2658fd60fc2601b28db0264f4d8ea98ac8d..950ac60497300a38cca784d9a4926d2a41f36402 100644
--- a/ash/shell/shell_delegate_impl.h
+++ b/ash/shell/shell_delegate_impl.h
@@ -55,6 +55,8 @@ class ShellDelegateImpl : public ash::ShellDelegate {
   ash::AccessibilityDelegate* CreateAccessibilityDelegate() override;
   ash::NewWindowDelegate* CreateNewWindowDelegate() override;
   ash::MediaDelegate* CreateMediaDelegate() override;
+  std::unique_ptr<PointerWatcherDelegate> CreatePointerWatcherDelegate()
+      override;
   ui::MenuModel* CreateContextMenu(ash::Shelf* shelf,
                                    const ash::ShelfItem* item) override;
   GPUSupport* CreateGPUSupport() override;