Fix missing HEADLESS_EXPORT defines

headless/lib/browser/headless_web_contents_impl.h

Index: headless/lib/browser/headless_web_contents_impl.h
diff --git a/headless/lib/browser/headless_web_contents_impl.h b/headless/lib/browser/headless_web_contents_impl.h
index 3346c64dca9bd04c460eec1bd37553b7192905f7..c966fca3601dbe6ac1d5bc3525b093f04931094d 100644
--- a/headless/lib/browser/headless_web_contents_impl.h
+++ b/headless/lib/browser/headless_web_contents_impl.h
@@ -13,6 +13,7 @@
 #include "content/public/browser/render_process_host_observer.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "headless/public/headless_devtools_target.h"
+#include "headless/public/headless_export.h"
 #include "headless/public/headless_web_contents.h"
 
 namespace content {
@@ -28,10 +29,12 @@ namespace headless {
 class HeadlessBrowserImpl;
 class WebContentsObserverAdapter;
 
-class HeadlessWebContentsImpl : public HeadlessWebContents,
-                                public HeadlessDevToolsTarget,
-                                public content::RenderProcessHostObserver,
-                                public content::WebContentsObserver {
+// Exported for tests.
+class HEADLESS_EXPORT HeadlessWebContentsImpl
+    : public HeadlessWebContents,
+      public HeadlessDevToolsTarget,
+      public content::RenderProcessHostObserver,
+      public content::WebContentsObserver {
  public:
   ~HeadlessWebContentsImpl() override;