src: make SIGPROF message a real warning

This commit replaces a fprintf() with a call to
ProcessEmitWarning().

Refs: https://github.com/nodejs/node/issues/12706
PR-URL: https://github.com/nodejs/node/pull/12709
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

src/signal_wrap.cc

@@ -92,7 +92,8 @@ class SignalWrap : public HandleWrap {
     if (signum == SIGPROF) {
       Environment* env = Environment::GetCurrent(args);
       if (env->inspector_agent()->IsStarted()) {
-        fprintf(stderr, "process.on(SIGPROF) is reserved while debugging\n");
+        ProcessEmitWarning(env,
+                           "process.on(SIGPROF) is reserved while debugging");
         return;
       }
     }

test/parallel/test-warn-sigprof.js

@@ -0,0 +1,17 @@
+'use strict';
+const common = require('../common');
+
+// The inspector attempts to start when Node starts. Once started, the inspector
+// warns on the use of a SIGPROF listener.
+
+common.skipIfInspectorDisabled();
+
+if (common.isWindows) {
+  common.skip('test does not apply to Windows');
+  return;
+}
+
+common.expectWarning('Warning',
+                     'process.on(SIGPROF) is reserved while debugging');
+
+process.on('SIGPROF', () => {});