Get headless_shell building on Windows.

headless/lib/headless_content_main_delegate.h

Index: headless/lib/headless_content_main_delegate.h
diff --git a/headless/lib/headless_content_main_delegate.h b/headless/lib/headless_content_main_delegate.h
index 2e7aafe7973b723b7bd088c5ee4dc45358bb03a7..90facbaf60f5f28680dd82ea9bae9e5a6f4a9e3c 100644
--- a/headless/lib/headless_content_main_delegate.h
+++ b/headless/lib/headless_content_main_delegate.h
@@ -30,7 +30,7 @@ class HeadlessContentMainDelegate : public content::ContentMainDelegate {
   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(); }