Adds a FlatDomTreeExtractor to headless

headless/public/util/dom_tree_extractor.h

Index: headless/public/util/dom_tree_extractor.h
diff --git a/headless/public/util/dom_tree_extractor.h b/headless/public/util/dom_tree_extractor.h
index dcd08bfe91726e0ed8b799a166ad487fae7b2b76..1ba9bf392000740b099ddb2b220a70ccb6b065a3 100644
--- a/headless/public/util/dom_tree_extractor.h
+++ b/headless/public/util/dom_tree_extractor.h
@@ -19,6 +19,7 @@ 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).
+// TODO(alexclarke): Remove in favor of one using DOM.getFlattenedDocument.
 class DomTreeExtractor {
  public:
   explicit DomTreeExtractor(HeadlessDevToolsClient* devtools_client);