Expose the full API for attaching devtools clients:

headless/lib/browser/headless_devtools_client_impl.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 HEADLESS_LIB_BROWSER_HEADLESS_DEVTOOLS_CLIENT_IMPL_H_
 #define HEADLESS_LIB_BROWSER_HEADLESS_DEVTOOLS_CLIENT_IMPL_H_
 
 #include <unordered_map>
 
 #include "base/memory/weak_ptr.h"
@@ skipping 97 matching lines @@
                    std::unique_ptr<base::Value> params,
                    base::Callback<void()> callback) override;
   void SendMessage(const char* method,
                    base::Callback<void(const base::Value&)> callback) override;
   void SendMessage(const char* method,
                    base::Callback<void()> callback) override;
   void RegisterEventHandler(
       const char* method,
       base::Callback<void(const base::Value&)> callback) override;
 
-  void AttachToHost(content::DevToolsAgentHost* agent_host);
+  bool AttachToHost(content::DevToolsAgentHost* agent_host);
+  void ForceAttachToHost(content::DevToolsAgentHost* agent_host);
   void DetachFromHost(content::DevToolsAgentHost* agent_host);
 
  private:
   // Contains a callback with or without a result parameter depending on the
   // message type.
   struct Callback {
     Callback();
     Callback(Callback&& other);
     explicit Callback(base::Callback<void()> callback);
     explicit Callback(base::Callback<void(const base::Value&)> callback);
@@ skipping 68 matching lines @@
   tracing::ExperimentalDomain tracing_domain_;
   scoped_refptr<base::SingleThreadTaskRunner> browser_main_thread_;
   base::WeakPtrFactory<HeadlessDevToolsClientImpl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(HeadlessDevToolsClientImpl);
 };
 
 }  // namespace headless
 
 #endif  // HEADLESS_LIB_BROWSER_HEADLESS_DEVTOOLS_CLIENT_IMPL_H_