[Tracing] Remove unnecessary memory allocation in runtime call stats.

src/counters.h

 // Copyright 2012 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_COUNTERS_H_
 #define V8_COUNTERS_H_
 
 #include "include/v8.h"
 #include "src/allocation.h"
 #include "src/base/platform/elapsed-timer.h"
@@ skipping 773 matching lines @@
   // parent.
   static void Leave(RuntimeCallStats* stats, RuntimeCallTimer* timer);
 
   // Set counter id for the innermost measurement. It can be used to refine
   // event kind when a runtime entry counter is too generic.
   static void CorrectCurrentCounterId(RuntimeCallStats* stats,
                                       CounterId counter_id);
 
   void Reset();
   V8_NOINLINE void Print(std::ostream& os);
-  V8_NOINLINE const char* Dump();
+  V8_NOINLINE std::string Dump();
 
   RuntimeCallStats() {
     Reset();
     in_use_ = false;
   }
 
   RuntimeCallTimer* current_timer() { return current_timer_; }
   bool InUse() { return in_use_; }
 
  private:
   std::stringstream buffer_;
-  std::unique_ptr<char[]> buffer_c_str_;
-  size_t len_ = 0;
   // Counter to track recursive time events.
   RuntimeCallTimer* current_timer_ = NULL;
   // Used to track nested tracing scopes.
   bool in_use_;
 };
 
 #define TRACE_RUNTIME_CALL_STATS(isolate, counter_name)                 \
   do {                                                                  \
     if (V8_UNLIKELY(TRACE_EVENT_RUNTIME_CALL_STATS_TRACING_ENABLED() || \
                     FLAG_runtime_call_stats)) {                         \
@@ skipping 448 matching lines @@
   }
 
   Isolate* isolate_ = nullptr;
   RuntimeCallTimer timer_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COUNTERS_H_