Make GCTracer not reentrant.

src/heap/gc-tracer.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_GC_TRACER_H_
 #define V8_HEAP_GC_TRACER_H_
 
 #include "src/base/platform/platform.h"
 
 namespace v8 {
@@ skipping 246 matching lines @@
   typedef RingBuffer<ContextDisposalEvent, kRingBufferMaxSize>
       ContextDisposalEventBuffer;
 
   explicit GCTracer(Heap* heap);
 
   // Start collecting data.
   void Start(GarbageCollector collector, const char* gc_reason,
              const char* collector_reason);
 
   // Stop collecting data and print results.
-  void Stop();
+  void Stop(GarbageCollector collector);
 
   // Log an allocation throughput event.
   void AddNewSpaceAllocationTime(double duration, intptr_t allocation_in_bytes);
 
   void AddContextDisposalTime(double time);
 
   // Log an incremental marking step.
   void AddIncrementalMarkingStep(double duration, intptr_t bytes);
 
   // Log time spent in marking.
@@ skipping 134 matching lines @@
   // different field for that.
   // TODO(hpayer): This timer right now just holds the sweeping time
   // of the initial atomic sweeping pause. Make sure that it accumulates
   // all sweeping operations performed on the main thread.
   double cumulative_sweeping_duration_;
 
   // Holds the new space top pointer recorded at the end of the last garbage
   // collection.
   intptr_t new_space_top_after_gc_;
 
+  // Counts how many tracers were started without stopping.
+  int start_counter_;
+
   DISALLOW_COPY_AND_ASSIGN(GCTracer);
 };
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_GC_TRACER_H_