src: move node_trace_writer/buffer.h to agent.cc

The headers node_trace_writer.h and node_trace_buffer.h are not used in
agent.h but are more of an implementation detail of agent.cc.

This commit suggests moving the inclusion of these headers to agent.cc.

PR-URL: #15598
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/tracing/agent.cc

@@ -2,6 +2,8 @@
 
 #include <sstream>
 #include <string>
+#include "tracing/node_trace_buffer.h"
+#include "tracing/node_trace_writer.h"
 
 #include "env-inl.h"
 

src/tracing/agent.h

@@ -1,15 +1,16 @@
 #ifndef SRC_TRACING_AGENT_H_
 #define SRC_TRACING_AGENT_H_
 
+#include "libplatform/v8-tracing.h"
 #include "node_platform.h"
-#include "tracing/node_trace_buffer.h"
-#include "tracing/node_trace_writer.h"
 #include "uv.h"
 #include "v8.h"
 
 namespace node {
 namespace tracing {
 
+using v8::platform::tracing::TracingController;
+
 class Agent {
  public:
   Agent();