Adds a FlatDomTreeExtractor to headless

headless/BUILD.gn

Index: headless/BUILD.gn
diff --git a/headless/BUILD.gn b/headless/BUILD.gn
index d6cd10639cd5caf71a922adaeb1e5ad02d769994..7f155ee2e080e1c520781bb5822be8ac047a6ac9 100644
--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -207,6 +207,8 @@ static_library("headless_lib") {
               "public/util/expedited_dispatcher.h",
               "public/util/generic_url_request_job.cc",
               "public/util/generic_url_request_job.h",
+              "public/util/flat_dom_tree_extractor.cc",
+              "public/util/flat_dom_tree_extractor.h",
               "public/util/http_url_fetcher.cc",
               "public/util/http_url_fetcher.h",
               "public/util/in_memory_protocol_handler.cc",
@@ -321,6 +323,7 @@ test("headless_browsertests") {
     "lib/headless_devtools_client_browsertest.cc",
     "lib/headless_web_contents_browsertest.cc",
     "public/util/dom_tree_extractor_browsertest.cc",
+    "public/util/flat_dom_tree_extractor_browsertest.cc",
     "test/headless_browser_test.cc",
     "test/headless_browser_test.h",
     "test/headless_test_launcher.cc",