Blink Plugins: Remove Shadow DOM Plugin Placeholder remnants.

public/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("//tools/grit/grit_rule.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
@@ skipping 540 matching lines @@
     "web/WebPluginScriptForbiddenScope.h",
     "web/WebDataSource.h",
     "web/WebDevToolsFrontendClient.h",
     "web/WebNavigatorContentUtilsClient.h",
     "web/WebSelectElement.h",
     "web/WebGeolocationPosition.h",
     "web/WebExternalPopupMenu.h",
     "web/WebDeviceEmulationParams.h",
     "web/WebTestInterfaceFactory.h",
     "web/WebScriptController.h",
-    "web/WebPluginPlaceholder.h",
     "web/WebSpeechRecognitionResult.h",
     "web/WebCache.h",
     "web/WebSharedWorkerConnector.h",
     "web/WebContentSettingsClient.h",
     "web/WebDocumentType.h",
     "web/WebCompositionUnderline.h",
     "web/WebSpeechGrammar.h",
     "web/WebSocketClient.h",
     "web/WebDevToolsAgentClient.h",
     "web/WebSerializedScriptValue.h",
@@ skipping 75 matching lines @@
 grit("image_resources") {
   output_dir = "$root_gen_dir/blink/public/resources"
   use_qualified_include = true
   source = "blink_image_resources.grd"
   outputs = [
     "grit/blink_image_resources.h",
     "blink_image_resources_100_percent.pak",
     "blink_image_resources_200_percent.pak",
   ]
 }