Fix missing HEADLESS_EXPORT defines

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 cb33f226d178bf684b7c7f54894b97bf07381b36..0f5c61e8f8e3fdc86ddb1fc005c45ded4438d1e8 100644
--- a/headless/lib/headless_content_main_delegate.h
+++ b/headless/lib/headless_content_main_delegate.h
@@ -13,6 +13,7 @@
 #include "content/public/app/content_main_delegate.h"
 #include "headless/lib/browser/headless_platform_event_source.h"
 #include "headless/lib/headless_content_client.h"
+#include "headless/public/headless_export.h"
 
 namespace base {
 class CommandLine;
@@ -23,7 +24,9 @@ namespace headless {
 class HeadlessBrowserImpl;
 class HeadlessContentBrowserClient;
 
-class HeadlessContentMainDelegate : public content::ContentMainDelegate {
+// Exported for tests.
+class HEADLESS_EXPORT HeadlessContentMainDelegate
+    : public content::ContentMainDelegate {
  public:
   explicit HeadlessContentMainDelegate(
       std::unique_ptr<HeadlessBrowserImpl> browser);