Expose the full API for attaching devtools clients:

headless/lib/browser/headless_devtools_client_impl.cc

 // 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.
 
 #include "headless/lib/browser/headless_devtools_client_impl.h"
 
 #include <memory>
 
 #include "base/bind.h"
 #include "base/json/json_reader.h"
@@ skipping 51 matching lines @@
       security_domain_(this),
       service_worker_domain_(this),
       target_domain_(this),
       tracing_domain_(this),
       browser_main_thread_(content::BrowserThread::GetTaskRunnerForThread(
           content::BrowserThread::UI)),
       weak_ptr_factory_(this) {}
 
 HeadlessDevToolsClientImpl::~HeadlessDevToolsClientImpl() {}
 
-void HeadlessDevToolsClientImpl::AttachToHost(
+bool HeadlessDevToolsClientImpl::AttachToHost(
+    content::DevToolsAgentHost* agent_host) {
+  DCHECK(!agent_host_);
+  if (agent_host->AttachClient(this)) {
+    agent_host_ = agent_host;
+    return true;
+  }
+  return false;
+}
+
+void HeadlessDevToolsClientImpl::ForceAttachToHost(
     content::DevToolsAgentHost* agent_host) {
   DCHECK(!agent_host_);
   agent_host_ = agent_host;
-  agent_host_->AttachClient(this);
+  agent_host_->ForceAttachClient(this);
 }
 
 void HeadlessDevToolsClientImpl::DetachFromHost(
     content::DevToolsAgentHost* agent_host) {
   DCHECK_EQ(agent_host_, agent_host);
   if (!renderer_crashed_)
     agent_host_->DetachClient(this);
   agent_host_ = nullptr;
   pending_messages_.clear();
 }
@@ skipping 282 matching lines @@
 HeadlessDevToolsClientImpl::Callback::Callback(
     base::Callback<void(const base::Value&)> callback)
     : callback_with_result(callback) {}
 
 HeadlessDevToolsClientImpl::Callback::~Callback() {}
 
 HeadlessDevToolsClientImpl::Callback& HeadlessDevToolsClientImpl::Callback::
 operator=(Callback&& other) = default;
 
 }  // namespace headless