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 cfe2ce00e8ee1a80dbb20d9837b7b3346b333cc6..c6c1577102a0b3ceb6e02e1bb0fabd47a6d30125 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",
@@ -319,6 +321,7 @@ 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",