Fix missing HEADLESS_EXPORT defines

headless/lib/browser/devtools_api/domain_h.template

 // This file is generated
 
 // Copyright (c) 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_DEVTOOLS_DOMAINS_{{domain.domain | camelcase_to_hacker_style | upper}}_H_
 #define HEADLESS_PUBLIC_DEVTOOLS_DOMAINS_{{domain.domain | camelcase_to_hacker_style | upper}}_H_
 
 #include "base/callback.h"
@@ skipping 32 matching lines @@
     {# If the command has no return value, generate a convenience method that #}
     {# accepts a base::Callback<void()> together with the parameters object. #}
     {% if not command.get("returns", []) %}
   void {{method_name}}(std::unique_ptr<{{method_name}}Params> params, base::Callback<void()> callback);
     {% endif %}
   {% endif %}
 {% endmacro %}
 
 namespace headless {
 namespace {{domain.domain | camelcase_to_hacker_style}} {
-class ExperimentalDomain;
-class ExperimentalObserver;
+class HEADLESS_EXPORT ExperimentalDomain;
+class HEADLESS_EXPORT ExperimentalObserver;
 {% if "events" in domain %}
 
 class HEADLESS_EXPORT ExperimentalObserver {
  public:
   virtual ~ExperimentalObserver() {}
   {% for event in domain.events %}
     {% if event.description %}
   // {{event.description}}
     {% endif %}
   virtual void On{{event.name | to_title_case}}(const {{event.name | to_title_case}}Params& params) {}
@@ skipping 92 matching lines @@
   {% endfor %}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ExperimentalDomain);
 };
 
 }  // namespace {{domain.domain | camelcase_to_hacker_style}}
 }  // namespace headless
 
 #endif  // HEADLESS_PUBLIC_DEVTOOLS_DOMAINS_{{domain.domain | camelcase_to_hacker_style | upper}}_H_