mus: Add PointerWatcher for passively observing mouse and touch events

components/mus/public/cpp/lib/window_tree_client_impl.h

Index: components/mus/public/cpp/lib/window_tree_client_impl.h
diff --git a/components/mus/public/cpp/lib/window_tree_client_impl.h b/components/mus/public/cpp/lib/window_tree_client_impl.h
index a63d417ae5db6ea051c5265d4bc279476dfe6c4b..491d5896e5869c6221d895f9ab44163875a4e575 100644
--- a/components/mus/public/cpp/lib/window_tree_client_impl.h
+++ b/components/mus/public/cpp/lib/window_tree_client_impl.h
@@ -167,6 +167,7 @@ class WindowTreeClientImpl : public WindowTreeConnection,
   const std::set<Window*>& GetRoots() override;
   Window* GetFocusedWindow() override;
   void ClearFocus() override;
+  void SetEventObserver(mojom::EventMatcherPtr matcher) override;
   Window* NewWindow(const Window::SharedProperties* properties) override;
   Window* NewTopLevelWindow(
       const Window::SharedProperties* properties) override;
@@ -299,6 +300,11 @@ class WindowTreeClientImpl : public WindowTreeConnection,
       window_manager_internal_;
   mojom::WindowManagerClientAssociatedPtr window_manager_internal_client_;
 
+  bool has_event_observer_ = false;
+
+  // Monotonically increasing ID for event observers.
+  uint32_t event_observer_id_ = 0u;
+
   DISALLOW_COPY_AND_ASSIGN(WindowTreeClientImpl);
 };