[DevTools] Migrate from AgentStateCallbacks to DevToolsAgentHostObserver.

content/public/browser/devtools_agent_host_observer.h

+// 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.
+
+#ifndef CONTENT_PUBLIC_BROWSER_DEVTOOLS_AGENT_HOST_OBSERVER_H_
+#define CONTENT_PUBLIC_BROWSER_DEVTOOLS_AGENT_HOST_OBSERVER_H_
+
+#include "content/common/content_export.h"
+
+namespace content {
+
+class DevToolsAgentHost;
+
+// Observer API notifies interested parties about changes in DevToolsAgentHosts.
+class CONTENT_EXPORT DevToolsAgentHostObserver {
+ public:
+  virtual ~DevToolsAgentHostObserver() {}
+
+  virtual void DevToolsAgentHostAttached(DevToolsAgentHost* agent_host) {}
+  virtual void DevToolsAgentHostDetached(DevToolsAgentHost* agent_host) {}
+};
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_BROWSER_DEVTOOLS_AGENT_HOST_OBSERVER_H_