Add a custom element to own structure of shadow DOM plugin placeholders.

Source/core/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/ui.gni")
 import("//third_party/WebKit/Source/bindings/bindings.gni")
 import("//third_party/WebKit/Source/bindings/core/v8/generated.gni")
 import("//third_party/WebKit/Source/bindings/modules/modules.gni")
 import("//third_party/WebKit/Source/bindings/modules/v8/generated.gni")
 import("//third_party/WebKit/Source/bindings/scripts/scripts.gni")
@@ skipping 899 matching lines @@
 # You can use Blink-in-JS in your local experiment, but don't ship it.
 # crbug.com/341031
 action("make_core_generated_private_script") {
   visibility = []  # Allow re-assignment of list.
   visibility = [ ":make_core_generated" ]
   script = "../build/scripts/make_private_script_source.py"
 
   inputs = [
     "../bindings/core/v8/PrivateScriptRunner.js",
     "html/HTMLMarqueeElement.js",
+    "html/shadow/PluginPlaceholderElement.js",
     "xml/DocumentXMLTreeViewer.js",
   ]
   outputs = [
     "$blink_core_output_dir/PrivateScriptSources.h",
   ]
 
   args = rebase_path(outputs, root_build_dir)
   args += rebase_path(inputs, root_build_dir)
 
   deps = make_core_generated_deps
@@ skipping 134 matching lines @@
     "$blink_core_output_dir/{{source_name_part}}.h",
   ]
   args = [
     "{{source}}",
     rel_blink_core_gen_dir,
     bison_exe,
   ]
 
   deps = make_core_generated_deps
 }