Adding Flush to AutoRollLogger

Summary:
During running AutoRollLoggerTest on FreeBSD we have found out that AutoRollLogger is not flushing correctly (test fails on FreeBSD)
This diff add Flush to AutoRollLogger to fix this problem

Test Plan:
[My machine] make all check ( all tests pass)
[FreeBSD VM] running AutoRollLoggerTest ( all tests pass )

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D33633

util/auto_roll_logger.h

@@ -56,6 +56,12 @@ class AutoRollLogger : public Logger {
     return logger_->GetLogFileSize();
   }
 
+  void Flush() {
+    if (logger_) {
+      logger_->Flush();
+    }
+  }
+
   virtual ~AutoRollLogger() {
   }
 

util/auto_roll_logger_test.cc

@@ -308,6 +308,7 @@ TEST(AutoRollLoggerTest, LogHeaderTest) {
     Info(&logger, (kSampleMessage + ":LogHeaderTest line %d").c_str(), i);
     ++i;
   }
+  LogFlush(&logger);
 
   // verify that the new log contains all the header logs
   std::stringstream ssbuf;