Add a mechanism to extend a libevent based message pump.

message_loop/message_pump_libevent.h

Index: message_loop/message_pump_libevent.h
diff --git a/message_loop/message_pump_libevent.h b/message_loop/message_pump_libevent.h
index 8b815aea588294002662dddb2c83f873e6240d6f..d35d1bdb1308cd9b7536e774c09d6da74587dcaa 100644
--- a/message_loop/message_pump_libevent.h
+++ b/message_loop/message_pump_libevent.h
@@ -37,6 +37,18 @@ class BASE_EXPORT MessagePumpLibevent : public MessagePump {
     virtual ~IOObserver() {}
   };
 
+  // Called to poll for events during each loop.
+  // Should return true if any events were processed.
+  class EventSource {
+   public:
+    EventSource() {}
+
+    virtual bool Poll() = 0;
+
+   protected:
+    virtual ~EventSource() {}
+  };
+
   // Used with WatchFileDescriptor to asynchronously monitor the I/O readiness
   // of a file descriptor.
   class Watcher {
@@ -122,6 +134,9 @@ class BASE_EXPORT MessagePumpLibevent : public MessagePump {
   void AddIOObserver(IOObserver* obs);
   void RemoveIOObserver(IOObserver* obs);
 
+  void SetEventSource(EventSource* event_source);
+  void ClearEventSource();
+
   // MessagePump methods:
   void Run(Delegate* delegate) override;
   void Quit() override;
@@ -134,6 +149,8 @@ class BASE_EXPORT MessagePumpLibevent : public MessagePump {
   void WillProcessIOEvent();
   void DidProcessIOEvent();
 
+  bool PollEventSource();
+
   // Risky part of constructor.  Returns true on success.
   bool Init();
 
@@ -169,6 +186,7 @@ class BASE_EXPORT MessagePumpLibevent : public MessagePump {
   event* wakeup_event_;
 
   ObserverList<IOObserver> io_observers_;
+  EventSource* event_source_;
   ThreadChecker watch_file_descriptor_caller_checker_;
   DISALLOW_COPY_AND_ASSIGN(MessagePumpLibevent);
 };