[heap] Implement concurrent marking boilerplate.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
@@ skipping 1413 matching lines @@
     "src/globals.h",
     "src/handles-inl.h",
     "src/handles.cc",
     "src/handles.h",
     "src/heap-symbols.h",
     "src/heap/array-buffer-tracker-inl.h",
     "src/heap/array-buffer-tracker.cc",
     "src/heap/array-buffer-tracker.h",
     "src/heap/code-stats.cc",
     "src/heap/code-stats.h",
+    "src/heap/concurrent-marking.cc",
+    "src/heap/concurrent-marking.h",
     "src/heap/embedder-tracing.cc",
     "src/heap/embedder-tracing.h",
     "src/heap/gc-idle-time-handler.cc",
     "src/heap/gc-idle-time-handler.h",
     "src/heap/gc-tracer.cc",
     "src/heap/gc-tracer.h",
     "src/heap/heap-inl.h",
     "src/heap/heap.cc",
     "src/heap/heap.h",
     "src/heap/incremental-marking-inl.h",
@@ skipping 1680 matching lines @@
   ]
 
   configs = [
     ":external_config",
     ":internal_config_base",
   ]
 }
 
 v8_fuzzer("wasm_compile_fuzzer") {
 }