Add plugins::TestPluginPlaceholder class and allow its use in Blink layout tests.

content/shell/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/features.gni")
 import("//build/config/ui.gni")
 import("//tools/grit/repack.gni")
 import("//tools/grit/grit_rule.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
@@ skipping 198 matching lines @@
     "renderer/test_runner/mock_webrtc_peer_connection_handler.cc",
     "renderer/test_runner/mock_webrtc_peer_connection_handler.h",
     "renderer/test_runner/spell_check_client.cc",
     "renderer/test_runner/spell_check_client.h",
     "renderer/test_runner/test_common.cc",
     "renderer/test_runner/test_common.h",
     "renderer/test_runner/test_interfaces.cc",
     "renderer/test_runner/test_interfaces.h",
     "renderer/test_runner/test_plugin.cc",
     "renderer/test_runner/test_plugin.h",
+    "renderer/test_runner/test_plugin_placeholder.cc",
+    "renderer/test_runner/test_plugin_placeholder.h",
     "renderer/test_runner/test_runner.cc",
     "renderer/test_runner/test_runner.h",
     "renderer/test_runner/text_input_controller.cc",
     "renderer/test_runner/text_input_controller.h",
     "renderer/test_runner/web_ax_object_proxy.cc",
     "renderer/test_runner/web_ax_object_proxy.h",
     "renderer/test_runner/web_content_settings.cc",
     "renderer/test_runner/web_content_settings.h",
     "renderer/test_runner/web_frame_test_proxy.h",
     "renderer/test_runner/web_task.cc",
@@ skipping 347 matching lines @@
 
     deps = [
       "//base",
       "//components/crash/tools:crash_service",
     ]
 
     configs -= [ "//build/config/win:console" ]
     configs += [ "//build/config/win:windowed" ]
   }
 }