[tracing] Implement Add/RemoveTraceStateObserver for default platform.

src/libplatform/default-platform.h

Index: src/libplatform/default-platform.h
diff --git a/src/libplatform/default-platform.h b/src/libplatform/default-platform.h
index 0fd7e5ad890a3090ab794c9d5c734a6ea29f45f3..7d38af91bdb5f8e77c489ee8b1f7173c5edc48ae 100644
--- a/src/libplatform/default-platform.h
+++ b/src/libplatform/default-platform.h
@@ -7,6 +7,7 @@
 
 #include <functional>
 #include <map>
+#include <memory>
 #include <queue>
 #include <vector>
 
@@ -61,6 +62,9 @@ class DefaultPlatform : public Platform {
                                 const char* name, uint64_t handle) override;
   void SetTracingController(tracing::TracingController* tracing_controller);
 
+  void AddTraceStateObserver(TraceStateObserver* observer) override;
+  void RemoveTraceStateObserver(TraceStateObserver* observer) override;
+
  private:
   static const int kMaxThreadPoolSize;
 
@@ -79,7 +83,7 @@ class DefaultPlatform : public Platform {
            std::priority_queue<DelayedEntry, std::vector<DelayedEntry>,
                                std::greater<DelayedEntry> > >
       main_thread_delayed_queue_;
-  tracing::TracingController* tracing_controller_;
+  std::unique_ptr<tracing::TracingController> tracing_controller_;
 
   DISALLOW_COPY_AND_ASSIGN(DefaultPlatform);
 };