mus: Add PointerWatcher for passively observing mouse and touch events

content/renderer/mus/compositor_mus_connection.cc

Index: content/renderer/mus/compositor_mus_connection.cc
diff --git a/content/renderer/mus/compositor_mus_connection.cc b/content/renderer/mus/compositor_mus_connection.cc
index be10408a2f12339c2c04d9a1d4a564f516088e3c..8569d1836e93f3d1c99584d5a582e9b3fdc8103b 100644
--- a/content/renderer/mus/compositor_mus_connection.cc
+++ b/content/renderer/mus/compositor_mus_connection.cc
@@ -116,6 +116,10 @@ void CompositorMusConnection::OnEmbed(mus::Window* root) {
   }
 }
 
+void CompositorMusConnection::OnEventObserved(const ui::Event& event) {
+  // Compositor does not use SetEventObserver().
+}
+
 void CompositorMusConnection::OnWindowInputEvent(
     mus::Window* window,
     const ui::Event& event,