[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 670969863e70362fb98ca8da00b3aee3fdd37a18..06cc873f799c2f4b61cad3096303e00a25f35f13 100644
--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -137,6 +137,7 @@ devtools_domains = [
   "device_orientation",
   "dom",
   "dom_debugger",
+  "dom_snapshot",
   "dom_storage",
   "emulation",
   "heap_profiler",
@@ -282,8 +283,6 @@ 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",
@@ -578,7 +577,6 @@ test("headless_browsertests") {
     "lib/headless_devtools_client_browsertest.cc",
     "lib/headless_web_contents_browsertest.cc",
     "lib/virtual_time_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",