[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 a038ce3cf09daefa81ad761cf81db79ace9f7a35..3dcace5c2b41405e58f08afcd2a14cf58bbbac9c 100644
--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -139,6 +139,7 @@ devtools_domains = [
   "device_orientation",
   "dom",
   "dom_debugger",
+  "dom_snapshot",
   "dom_storage",
   "emulation",
   "heap_profiler",
@@ -271,14 +272,10 @@ 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",
     "public/util/expedited_dispatcher.h",
-    "public/util/flat_dom_tree_extractor.cc",
-    "public/util/flat_dom_tree_extractor.h",
     "public/util/generic_url_request_job.cc",
     "public/util/generic_url_request_job.h",
     "public/util/http_url_fetcher.cc",
@@ -512,8 +509,6 @@ 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",
     "test/headless_test_launcher.cc",