Fix missing HEADLESS_EXPORT defines

headless/lib/browser/headless_browser_impl.h

Index: headless/lib/browser/headless_browser_impl.h
diff --git a/headless/lib/browser/headless_browser_impl.h b/headless/lib/browser/headless_browser_impl.h
index 864e49c1c17cc666a7ed5a9ee757e8ffb256c9b6..d54ecfd77eacd65e9f32707b0568fee1fa44d4cf 100644
--- a/headless/lib/browser/headless_browser_impl.h
+++ b/headless/lib/browser/headless_browser_impl.h
@@ -16,6 +16,7 @@
 #include "content/public/browser/web_contents.h"
 #include "headless/lib/browser/headless_devtools_manager_delegate.h"
 #include "headless/lib/browser/headless_web_contents_impl.h"
+#include "headless/public/headless_export.h"
 
 #if defined(USE_AURA)
 #include "headless/lib/browser/headless_window_tree_host.h"
@@ -32,7 +33,8 @@ namespace headless {
 class HeadlessBrowserContextImpl;
 class HeadlessBrowserMainParts;
 
-class HeadlessBrowserImpl : public HeadlessBrowser {
+// Exported for tests.
+class HEADLESS_EXPORT HeadlessBrowserImpl : public HeadlessBrowser {
  public:
   HeadlessBrowserImpl(
       const base::Callback<void(HeadlessBrowser*)>& on_start_callback,