headless: Implement DevTools events

headless/public/internal/message_dispatcher.h

Index: headless/public/internal/message_dispatcher.h
diff --git a/headless/public/internal/message_dispatcher.h b/headless/public/internal/message_dispatcher.h
index 32352f4804222f92ff6664e40578ee997e340fb7..ea939782b84df8766424a7fb62e2e7f71258bebf 100644
--- a/headless/public/internal/message_dispatcher.h
+++ b/headless/public/internal/message_dispatcher.h
@@ -23,6 +23,10 @@ class MessageDispatcher {
       base::Callback<void(const base::Value&)> callback) = 0;
   virtual void SendMessage(const char* method,
                            base::Callback<void()> callback) = 0;
+
+  virtual void RegisterEventHandler(
+      const char* method,
+      base::Callback<void(const base::Value&)> callback) = 0;
 };
 
 }  // namespace internal