[profiler] Introduce lightweight mode for Rutime Call Stats collection.

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/atomic-utils.h"
 #include "src/base/platform/elapsed-timer.h"
 #include "src/base/platform/time.h"
 #include "src/builtins/builtins.h"
 #include "src/globals.h"
 #include "src/isolate.h"
 #include "src/objects.h"
 #include "src/runtime/runtime.h"
 #include "src/tracing/trace-event.h"
 #include "src/tracing/traced-value.h"
+#include "src/tracing/tracing-category-observer.h"
 
 namespace v8 {
 namespace internal {
 
 // StatsCounters is an interface for plugging into external
 // counters for monitoring.  Counters can be looked up and
 // manipulated by name.
 
 class StatsTable {
  public:
@@ skipping 470 matching lines @@
   RuntimeCallCounter* counter() { return counter_; }
   base::ElapsedTimer timer() { return timer_; }
   RuntimeCallTimer* parent() const { return parent_.Value(); }
 
  private:
   friend class RuntimeCallStats;
 
   inline void Start(RuntimeCallCounter* counter, RuntimeCallTimer* parent) {
     counter_ = counter;
     parent_.SetValue(parent);
-    timer_.Start();
+    if (FLAG_runtime_stats !=
+        v8::tracing::TracingCategoryObserver::ENABLED_BY_SAMPLING) {
+      timer_.Start();
+    }
   }
 
   inline RuntimeCallTimer* Stop() {
+    if (!timer_.IsStarted()) return parent();
     base::TimeDelta delta = timer_.Elapsed();
     timer_.Stop();
     counter_->count++;
     counter_->time += delta;
     if (parent()) {
       // Adjust parent timer so that it does not include sub timer's time.
       parent()->counter_->time -= delta;
     }
     return parent();
   }
@@ skipping 779 matching lines @@
   }
 
   Isolate* isolate_ = nullptr;
   RuntimeCallTimer timer_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COUNTERS_H_