[DevTools] Implement Target.setDiscoverTargets method.

content/public/browser/devtools_agent_host_observer.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "content/public/browser/devtools_agent_host_observer.h"
+
+namespace content {
+
+DevToolsAgentHostObserver::~DevToolsAgentHostObserver() {
+}
+
+bool DevToolsAgentHostObserver::ShouldForceDevToolsAgentHostCreation() {
+  return false;
+}
+
+void DevToolsAgentHostObserver::DevToolsAgentHostCreated(
+    DevToolsAgentHost* agent_host) {
+}
+
+void DevToolsAgentHostObserver::DevToolsAgentHostAttached(
+    DevToolsAgentHost* agent_host) {
+}
+
+void DevToolsAgentHostObserver::DevToolsAgentHostDetached(
+    DevToolsAgentHost* agent_host) {
+}
+
+void DevToolsAgentHostObserver::DevToolsAgentHostDestroyed(
+    DevToolsAgentHost* agent_host) {
+}
+
+}  // namespace content