Fix missing HEADLESS_EXPORT defines

headless/public/util/flat_dom_tree_extractor.h

Index: headless/public/util/flat_dom_tree_extractor.h
diff --git a/headless/public/util/flat_dom_tree_extractor.h b/headless/public/util/flat_dom_tree_extractor.h
index 14de70d8572e3911a44d7035cfc8fa2933edec12..867c4594c75f646d175714bdcb7f2830394bbe3e 100644
--- a/headless/public/util/flat_dom_tree_extractor.h
+++ b/headless/public/util/flat_dom_tree_extractor.h
@@ -11,6 +11,7 @@
 #include "base/macros.h"
 #include "headless/public/devtools/domains/css.h"
 #include "headless/public/devtools/domains/dom.h"
+#include "headless/public/headless_export.h"
 
 namespace headless {
 class HeadlessDevToolsClient;
@@ -19,7 +20,7 @@ class HeadlessDevToolsClient;
 // addition, it also extracts details of bounding boxes and layout text (NB the
 // exact layout should not be regarded as stable, it's subject to change without
 // notice).
-class FlatDomTreeExtractor {
+class HEADLESS_EXPORT FlatDomTreeExtractor {
  public:
   explicit FlatDomTreeExtractor(HeadlessDevToolsClient* devtools_client);
   ~FlatDomTreeExtractor();
@@ -27,7 +28,7 @@ class FlatDomTreeExtractor {
   using NodeId = int;
   using Index = size_t;
 
-  class DomTree {
+  class HEADLESS_EXPORT DomTree {
    public:
     DomTree();
     DomTree(DomTree&& other);