Add a mechanism to extend a libevent based message pump.

message_loop/message_pump_libevent_unittest.cc

Index: message_loop/message_pump_libevent_unittest.cc
diff --git a/message_loop/message_pump_libevent_unittest.cc b/message_loop/message_pump_libevent_unittest.cc
index e911905abd7b489b47254a7195f6315701b422e9..16aba35353d2e63c6652659006b25eb01aa2ca80 100644
--- a/message_loop/message_pump_libevent_unittest.cc
+++ b/message_loop/message_pump_libevent_unittest.cc
@@ -271,6 +271,41 @@ TEST_F(MessagePumpLibeventTest, QuitWatcher) {
                                       Owned(watcher.release())));
 }
 
+class FakeEventSource : public MessagePumpLibevent::EventSource {
+ public:
+  FakeEventSource() : work_units_(0u) {}
+  ~FakeEventSource() override {}
+
+  void set_work_units(uint32_t work_units) { work_units_ = work_units; }
+  uint32_t work_units() const { return work_units_; }
+
+  bool Poll() override {
+    if (!work_units_)
+      return false;
+    work_units_--;
+    return true;
+  }
+
+ private:
+  uint32_t work_units_;
+};
+
+TEST_F(MessagePumpLibeventTest, PollEventSource) {
+  FakeEventSource event_source;
+
+  // Verify event source gets polled repeatedly until no work remains.
+  MessageLoopForUI::current()->SetEventSource(&event_source);
+  event_source.set_work_units(5u);
+  ui_loop_->RunUntilIdle();
+  EXPECT_EQ(0u, event_source.work_units());
+
+  // After removing the event source, it should no longer be polled.
+  MessageLoopForUI::current()->ClearEventSource();
+  event_source.set_work_units(5u);
+  ui_loop_->RunUntilIdle();
+  EXPECT_EQ(5u, event_source.work_units());
+}
+
 }  // namespace
 
 }  // namespace base