mus: Add PointerWatcher for passively observing mouse and touch events

components/mus/public/cpp/window_tree_delegate.h

Index: components/mus/public/cpp/window_tree_delegate.h
diff --git a/components/mus/public/cpp/window_tree_delegate.h b/components/mus/public/cpp/window_tree_delegate.h
index c20b5917622b6da041c50c7903ae61db5e6bace2..ff16625117b5ce41ba19b2725b21e0ec072e3c2d 100644
--- a/components/mus/public/cpp/window_tree_delegate.h
+++ b/components/mus/public/cpp/window_tree_delegate.h
@@ -10,6 +10,10 @@
 #include "components/mus/public/interfaces/window_tree.mojom.h"
 #include "services/shell/public/interfaces/interface_provider.mojom.h"
 
+namespace ui {
+class Event;
+}
+
 namespace mus {
 
 class Window;
@@ -47,6 +51,10 @@ class WindowTreeDelegate {
   // have been destroyed. |connection| is no longer valid after this call.
   virtual void OnConnectionLost(WindowTreeConnection* connection) = 0;
 
+  // Called when the WindowTreeConnection receives an input event observed via
+  // SetEventObserver().
+  virtual void OnEventObserved(const ui::Event& event) = 0;
+
  protected:
   virtual ~WindowTreeDelegate() {}
 };