Add a utility class for extracting details of the DOM

headless/BUILD.gn

Index: headless/BUILD.gn
diff --git a/headless/BUILD.gn b/headless/BUILD.gn
index a8a01a301f6508613dc34ae61e4acdc156c4e948..c48f6563b53c98cc57779655c031a04848ba432b 100644
--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -204,6 +204,8 @@ static_library("headless_lib") {
               "public/util/deterministic_dispatcher.h",
               "public/util/deterministic_http_protocol_handler.cc",
               "public/util/deterministic_http_protocol_handler.h",
+              "public/util/dom_tree_extractor.cc",
+              "public/util/dom_tree_extractor.h",
               "public/util/error_reporter.cc",
               "public/util/error_reporter.h",
               "public/util/expedited_dispatcher.cc",
@@ -337,6 +339,7 @@ test("headless_browsertests") {
     "lib/headless_browser_context_browsertest.cc",
     "lib/headless_devtools_client_browsertest.cc",
     "lib/headless_web_contents_browsertest.cc",
+    "public/util/dom_tree_extractor_browsertest.cc",
     "test/headless_browser_test.cc",
     "test/headless_browser_test.h",
     "test/headless_test_launcher.cc",