[devtools] Add DOMSnapshot domain for dom+layout+style snapshots.

headless/BUILD.gn

Index: headless/BUILD.gn
diff --git a/headless/BUILD.gn b/headless/BUILD.gn
index 00841786f9a5ec51549d517134efa4d309db7944..fe407b824df744f3abd426939e0c8ec0c8f04425 100644
--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -270,8 +270,6 @@ component("headless") {
     "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",
@@ -505,7 +503,6 @@ 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",
     "public/util/flat_dom_tree_extractor_browsertest.cc",
     "test/headless_browser_test.cc",
     "test/headless_browser_test.h",