[Tracing] Fix inaccurate timer calculation in runtime statistics.

src/tracing/trace-event.cc

 // Copyright 2015 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.
 
 #include "src/tracing/trace-event.h"
 
 #include <string.h>
 
 #include "src/counters.h"
 #include "src/isolate.h"
@@ skipping 31 matching lines @@
 
 void CallStatsScopedTracer::Initialize(v8::internal::Isolate* isolate,
                                        const uint8_t* category_group_enabled,
                                        const char* name) {
   data_.isolate = isolate;
   data_.category_group_enabled = category_group_enabled;
   data_.name = name;
   p_data_ = &data_;
   RuntimeCallStats* table = isolate->counters()->runtime_call_stats();
   has_parent_scope_ = table->InUse();
-  if (!has_parent_scope_) table->Reset();
+  if (!has_parent_scope_ && table->current_timer() == NULL) table->Reset();
   v8::internal::tracing::AddTraceEvent(
       TRACE_EVENT_PHASE_BEGIN, category_group_enabled, name,
       v8::internal::tracing::kGlobalScope, v8::internal::tracing::kNoId,
       TRACE_EVENT_FLAG_NONE, v8::internal::tracing::kNoId);
 }
 
 }  // namespace tracing
 }  // namespace internal
 }  // namespace v8