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

runtime/vm/log.h

 // 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.
 
 #ifndef RUNTIME_VM_LOG_H_
 #define RUNTIME_VM_LOG_H_
 
 #include "vm/allocation.h"
 #include "vm/growable_array.h"
 #include "vm/os.h"
@@ skipping 34 matching lines @@
 
   // Current cursor.
   intptr_t cursor() const;
 
   // A logger that does nothing.
   static Log* NoOpLog();
 
  private:
   void TerminateString();
   void EnableManualFlush();
-  void DisableManualFlush();
+  void DisableManualFlush(const intptr_t cursor);
 
   // Returns false if we should drop log messages related to 'isolate'.
   static bool ShouldLogForIsolate(const Isolate* isolate);
 
   static Log noop_log_;
   LogPrinter printer_;
   intptr_t manual_flush_;
   MallocGrowableArray<char> buffer_;
 
   friend class LogBlock;
@@ skipping 22 matching lines @@
  private:
   void Initialize();
 
   Log* const log_;
   const intptr_t cursor_;
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_LOG_H_