Get headless_shell building on Windows.

headless/lib/headless_content_main_delegate.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_HEADLESS_CONTENT_MAIN_DELEGATE_H_
 #define HEADLESS_LIB_HEADLESS_CONTENT_MAIN_DELEGATE_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
@@ skipping 12 matching lines @@
   explicit HeadlessContentMainDelegate(
       std::unique_ptr<HeadlessBrowserImpl> browser);
   ~HeadlessContentMainDelegate() override;
 
   // content::ContentMainDelegate implementation:
   bool BasicStartupComplete(int* exit_code) override;
   void PreSandboxStartup() override;
   int RunProcess(
       const std::string& process_type,
       const content::MainFunctionParams& main_function_params) override;
-  void ZygoteForked() override;
+  void ZygoteForked();

[dvallet, 2017/02/13 03:20:24]

Please change to:
#if defined(OS_WIN)
  void ZygoteForked();
#else
  void ZygoteForked() override;
#endif

   content::ContentBrowserClient* CreateContentBrowserClient() override;
 
   HeadlessBrowserImpl* browser() const { return browser_.get(); }
 
  private:
   friend class HeadlessBrowserTest;
 
   static void InitializeResourceBundle();
 
   static HeadlessContentMainDelegate* GetInstance();
 
   std::unique_ptr<HeadlessContentBrowserClient> browser_client_;
   HeadlessContentClient content_client_;
   HeadlessPlatformEventSource platform_event_source_;
 
   std::unique_ptr<HeadlessBrowserImpl> browser_;
 
   DISALLOW_COPY_AND_ASSIGN(HeadlessContentMainDelegate);
 };
 
 }  // namespace headless
 
 #endif  // HEADLESS_LIB_HEADLESS_CONTENT_MAIN_DELEGATE_H_