src: reset StopTracingAgent() before platform teardown

This makes sure that `StopTracingAgent()` is always called
before tearing down the `tracing::Agent`,
since previously its destructor might have tried to access the
agent, which would be destroyed by the (earlier) `Dispose()` call.

PR-URL: #25472
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

src/node.cc

@@ -240,6 +240,7 @@ static struct {
  }
 
  void Dispose() {
+ StopTracingAgent();
  platform_->Shutdown();
  delete platform_;
  platform_ = nullptr;
@@ -579,7 +580,6 @@ static void WaitForInspectorDisconnect(Environment* env) {
 void Exit(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  WaitForInspectorDisconnect(env);
- v8_platform.StopTracingAgent();
  int code = args[0]->Int32Value(env->context()).FromMaybe(0);
  env->Exit(code);
 }
@@ -1468,7 +1468,6 @@ int Start(int argc, char** argv) {
  per_process::v8_initialized = true;
  const int exit_code =
  Start(uv_default_loop(), args, exec_args);
- v8_platform.StopTracingAgent();
  per_process::v8_initialized = false;
  V8::Dispose();