[DevTools] Implement Target.setDiscoverTargets method.

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 ~DevToolsAgentHostObserver();
 
-  virtual void DevToolsAgentHostAttached(DevToolsAgentHost* agent_host) {}
-  virtual void DevToolsAgentHostDetached(DevToolsAgentHost* agent_host) {}
+  // If observer returns |true|, DevToolsAgentHost instances are created
+  // (and reported in DevToolsAgentHostCreated) for every possible devtools
+  // target (e.g. WebContents).
+  virtual bool ShouldForceDevToolsAgentHostCreation();
+
+  // Called when DevToolsAgentHost was created and is ready to be used.
+  virtual void DevToolsAgentHostCreated(DevToolsAgentHost* agent_host);
+
+  // Called when client has attached to DevToolsAgentHost.
+  virtual void DevToolsAgentHostAttached(DevToolsAgentHost* agent_host);
+
+  // Called when client has detached from DevToolsAgentHost.
+  virtual void DevToolsAgentHostDetached(DevToolsAgentHost* agent_host);
+
+  // Called when DevToolsAgentHost was destroyed.
+  virtual void DevToolsAgentHostDestroyed(DevToolsAgentHost* agent_host);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_DEVTOOLS_AGENT_HOST_OBSERVER_H_