Add persisted preference for projection touch HUD

ash/shell_observer.h

Index: ash/shell_observer.h
diff --git a/ash/shell_observer.h b/ash/shell_observer.h
index 0bdd93472a943bce1ebd7219c301f8b0cf1091ea..060e30afc4b4a819a9c9e5cfbae19f0b4f17581a 100644
--- a/ash/shell_observer.h
+++ b/ash/shell_observer.h
@@ -32,6 +32,9 @@ class ASH_EXPORT ShellObserver {
   // Invoked when the shelf alignment in |root_window| is changed.
   virtual void OnShelfAlignmentChanged(aura::RootWindow* root_window) {}
 
+  // Invoked when the projection touch HUD is toggled.
+  virtual void OnTouchHudProjectionToggled(bool enabled) {}
+
  protected:
   virtual ~ShellObserver() {}
 };