Refactoring to make adding ink drop animations easier

ui/events/event_target.h

Index: ui/events/event_target.h
diff --git a/ui/events/event_target.h b/ui/events/event_target.h
index e8b14a6445851b3a5b2e00a9c5dae39c0a79da2e..21a84a4db6da1c7a211d2ebed3fcdc31a368ab97 100644
--- a/ui/events/event_target.h
+++ b/ui/events/event_target.h
@@ -77,9 +77,8 @@ class EVENTS_EXPORT EventTarget : public EventHandler {
   // Returns true if the event pre target list is empty.
   bool IsPreTargetListEmpty() const;
 
-  void set_target_handler(EventHandler* handler) {
-    target_handler_ = handler;
-  }
+  // Sets |target_handler| as |target_handler_| and returns the old handler.
+  EventHandler* SetTargetHandler(EventHandler* target_handler);
 
  protected:
   EventHandler* target_handler() { return target_handler_; }