More DCHECK() updates. A mixture of _EQ and _GE.

base/message_pump_libevent.cc

Index: base/message_pump_libevent.cc
diff --git a/base/message_pump_libevent.cc b/base/message_pump_libevent.cc
index 933d7950eacd331fb08bdb70e4dc3c54187a9206..28b4bfe91352434d9884ad9277a8811651ab9d9f 100644
--- a/base/message_pump_libevent.cc
+++ b/base/message_pump_libevent.cc
@@ -78,7 +78,7 @@ bool MessagePumpLibevent::FileDescriptorWatcher::StopWatchingFileDescriptor() {
 void MessagePumpLibevent::FileDescriptorWatcher::Init(event *e,
                                                       bool is_persistent) {
   DCHECK(e);
-  DCHECK(event_ == NULL);
+  DCHECK(!event_);
 
   is_persistent_ = is_persistent;
   event_ = e;