Make GCTracer not reentrant.

src/heap/gc-tracer.h

Index: src/heap/gc-tracer.h
diff --git a/src/heap/gc-tracer.h b/src/heap/gc-tracer.h
index 04dac62f0261fe6447eb5a7e6e71bbb5f57e8d4b..3ea1d72bf2c29a3f063cb76ac506f9e4f2ca29e4 100644
--- a/src/heap/gc-tracer.h
+++ b/src/heap/gc-tracer.h
@@ -264,7 +264,7 @@ class GCTracer {
              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);
@@ -419,6 +419,9 @@ class GCTracer {
   // collection.
   intptr_t new_space_top_after_gc_;
 
+  // Counts how many tracers were started without stopping.
+  int start_counter_;
+
   DISALLOW_COPY_AND_ASSIGN(GCTracer);
 };
 }