Expose the full API for attaching devtools clients:

headless/lib/browser/headless_web_contents_impl.h

 // Copyright 2015 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_WEB_CONTENTS_IMPL_H_
 #define HEADLESS_LIB_BROWSER_HEADLESS_WEB_CONTENTS_IMPL_H_
 
 #include <list>
 #include <memory>
 #include <string>
@@ skipping 40 matching lines @@
   static std::unique_ptr<HeadlessWebContentsImpl> CreateFromWebContents(
       content::WebContents* web_contents,
       HeadlessBrowserContextImpl* browser_context);
 
   // HeadlessWebContents implementation:
   void AddObserver(Observer* observer) override;
   void RemoveObserver(Observer* observer) override;
   HeadlessDevToolsTarget* GetDevToolsTarget() override;
 
   // HeadlessDevToolsTarget implementation:
-  void AttachClient(HeadlessDevToolsClient* client) override;
+  bool AttachClient(HeadlessDevToolsClient* client) override;
+  void ForceAttachClient(HeadlessDevToolsClient* client) override;
   void DetachClient(HeadlessDevToolsClient* client) override;
+  bool IsAttached() override;
 
   // RenderProcessHostObserver implementation:
   void RenderProcessExited(content::RenderProcessHost* host,
                            base::TerminationStatus status,
                            int exit_code) override;
   void RenderProcessHostDestroyed(content::RenderProcessHost* host) override;
 
   // content::WebContentsObserver implementation:
   void RenderFrameCreated(content::RenderFrameHost* render_frame_host) override;
 
@@ skipping 30 matching lines @@
       std::unordered_map<HeadlessWebContents::Observer*,
                          std::unique_ptr<WebContentsObserverAdapter>>;
   ObserverMap observer_map_;
 
   DISALLOW_COPY_AND_ASSIGN(HeadlessWebContentsImpl);
 };
 
 }  // namespace headless
 
 #endif  // HEADLESS_LIB_BROWSER_HEADLESS_WEB_CONTENTS_IMPL_H_