[VM logging] Fix nesting of log blocker (second attempt).

runtime/vm/log.cc

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/log.h"
 
 #include "vm/flags.h"
 #include "vm/isolate.h"
 #include "vm/thread.h"
 
@@ skipping 131 matching lines @@
   buffer_.Add('\0');
 }
 
 void Log::EnableManualFlush() {
   if (this == NoOpLog()) {
     return;
   }
   manual_flush_++;
 }
 
-void Log::DisableManualFlush() {
+void Log::DisableManualFlush(const intptr_t cursor) {
   if (this == NoOpLog()) {
     return;
   }
 
   manual_flush_--;
   ASSERT(manual_flush_ >= 0);
   if (manual_flush_ == 0) {
-    Flush();
+    Flush(cursor);
   }
 }
 
 void LogBlock::Initialize() {
   log_->EnableManualFlush();
 }
 
 LogBlock::~LogBlock() {
-  log_->Flush(cursor_);
-  log_->DisableManualFlush();
+  log_->DisableManualFlush(cursor_);
 }
 
 }  // namespace dart