events: Start changing EventHandler interface to not return a value.

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 5f0652140ce73968e536ded634ec1fe07db7b81f..405c65d3bc0fc9cad68e08a487bfd29e664a1e4f 100644
--- a/ui/base/events/event_handler.h
+++ b/ui/base/events/event_handler.h
@@ -32,7 +32,7 @@ class UI_EXPORT EventHandler {
   // 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 void OnEvent(Event* event);
 
   virtual EventResult OnKeyEvent(KeyEvent* event);