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

src/tracing/tracing-category-observer.h

 // Copyright 2016 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_TRACING_TRACING_CATEGORY_OBSERVER_H_
 #define V8_TRACING_TRACING_CATEGORY_OBSERVER_H_
 
 #include "include/v8-platform.h"
-#include "src/base/lazy-instance.h"
 
 namespace v8 {
 namespace tracing {
 
 class TracingCategoryObserver : public Platform::TraceStateObserver {
  public:
   enum Mode {
     ENABLED_BY_NATIVE = 1 << 0,
     ENABLED_BY_TRACING = 1 << 1,
+    ENABLED_BY_SAMPLING = 1 << 2,
   };
 
   static void SetUp();
   static void TearDown();
 
   // v8::Platform::TraceStateObserver
   void OnTraceEnabled() final;
   void OnTraceDisabled() final;
 
-  TracingCategoryObserver() {}
-  ~TracingCategoryObserver() {}
-
  private:
   static TracingCategoryObserver* instance_;
 };
 
 }  // namespace tracing
 }  // namespace v8
+
 #endif  // V8_TRACING_TRACING_CATEGORY_OBSERVER_H_