events: Dispatch all events through EventHandler::OnEvent from the dispatcher.

ui/base/events/event_handler.h

Index: ui/base/events/event_handler.h
diff --git a/ui/base/events/event_handler.h b/ui/base/events/event_handler.h
index 567474f0aa1919290cf3590004323f3f1a79ef05..5f0652140ce73968e536ded634ec1fe07db7b81f 100644
--- a/ui/base/events/event_handler.h
+++ b/ui/base/events/event_handler.h
@@ -28,6 +28,10 @@ class UI_EXPORT EventHandler {
   EventHandler();
   virtual ~EventHandler();
 
+  // This is called for all events. The default implementation routes the event
+  // to one of the event-specific callbacks (OnKeyEvent, OnMouseEvent etc.). If
+  // this is overridden, then normally, the override should chain into the
+  // default implementation for un-handled events.
   virtual EventResult OnEvent(Event* event);
 
   virtual EventResult OnKeyEvent(KeyEvent* event);