Expose the full API for attaching devtools clients:

headless/public/headless_devtools_target.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_PUBLIC_HEADLESS_DEVTOOLS_TARGET_H_
 #define HEADLESS_PUBLIC_HEADLESS_DEVTOOLS_TARGET_H_
 
 #include "base/macros.h"
 #include "headless/public/headless_export.h"
 
 namespace headless {
 class HeadlessDevToolsClient;
 
 // A target which can be controlled and inspected using DevTools.
 class HEADLESS_EXPORT HeadlessDevToolsTarget {
  public:
   HeadlessDevToolsTarget() {}
   virtual ~HeadlessDevToolsTarget() {}
 
   // Attach or detach a client to this target. A client must be attached in
   // order to send commands or receive notifications from the target.
   //
-  // A single client may be attached to at most one target at a time. Note that
-  // currently also only one client may be attached to a single target at a
-  // time.
+  // A single client may be attached to at most one target at a time. If
+  // the target already has a client attached, AttachClient will return false.
+  // ForceAttachClient will detach any existing connection before attaching
+  // |client|.
+  // Note that currently also only one client may be attached to a single target
+  // at a time.
   //
   // |client| must outlive this target.
-  virtual void AttachClient(HeadlessDevToolsClient* client) = 0;
+  virtual bool AttachClient(HeadlessDevToolsClient* client) = 0;
+  virtual void ForceAttachClient(HeadlessDevToolsClient* client) = 0;
   virtual void DetachClient(HeadlessDevToolsClient* client) = 0;
 
+  // Returns true if a devtools client is attached.
+  virtual bool IsAttached() = 0;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(HeadlessDevToolsTarget);
 };
 
 }  // namespace headless
 
 #endif  // HEADLESS_PUBLIC_HEADLESS_DEVTOOLS_TARGET_H_