[Tracing] Create TracingCategoryObserver in gin/.

gin/v8_platform.cc

Index: gin/v8_platform.cc
diff --git a/gin/v8_platform.cc b/gin/v8_platform.cc
index 945eee0cc3d9798730130ebbe44d3d5acb92d4f7..d334332592daee6e59cc76c4c55ea4993031e283 100644
--- a/gin/v8_platform.cc
+++ b/gin/v8_platform.cc
@@ -10,6 +10,7 @@
 #include "base/threading/worker_pool.h"
 #include "base/trace_event/trace_event.h"
 #include "gin/per_isolate_data.h"
+#include "v8/include/v8-tracing.h"
 
 namespace gin {
 
@@ -154,7 +155,11 @@ namespace {
 class EnabledStateObserverImpl final
     : public base::trace_event::TraceLog::EnabledStateObserver {
  public:
-  EnabledStateObserverImpl() = default;
+  EnabledStateObserverImpl() {
+    tracing_category_observer_ = v8::tracing::TracingCategoryObserver::Create();

[alph, 2016/10/31 22:48:55]

Why it belongs here? It should be an internal class in V8.

+    observers_.insert(reinterpret_cast<v8::Platform::TraceStateObserver*>(

[alph, 2016/10/31 22:48:55]

Reinterpret cast between interfaces won't work.

+        tracing_category_observer_.get()));
+  }
 
   void OnTraceLogEnabled() final {
     base::AutoLock lock(mutex_);
@@ -192,6 +197,8 @@ class EnabledStateObserverImpl final
  private:
   base::Lock mutex_;
   std::unordered_set<v8::Platform::TraceStateObserver*> observers_;
+  std::unique_ptr<v8::tracing::TracingCategoryObserver>
+      tracing_category_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(EnabledStateObserverImpl);
 };