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 146 matching lines @@
     "renderer/layout_test/layout_test_content_renderer_client.cc",
     "renderer/layout_test/layout_test_content_renderer_client.h",
     "renderer/layout_test/layout_test_render_frame_observer.cc",
     "renderer/layout_test/layout_test_render_frame_observer.h",
     "renderer/layout_test/layout_test_render_process_observer.cc",
     "renderer/layout_test/layout_test_render_process_observer.h",
     "renderer/layout_test/leak_detector.cc",
     "renderer/layout_test/leak_detector.h",
     "renderer/layout_test/test_media_stream_renderer_factory.cc",
     "renderer/layout_test/test_media_stream_renderer_factory.h",
+    "renderer/layout_test/test_plugin_placeholder.cc",
+    "renderer/layout_test/test_plugin_placeholder.h",
     "renderer/layout_test/test_video_frame_provider.cc",
     "renderer/layout_test/test_video_frame_provider.h",
     "renderer/shell_content_renderer_client.cc",
     "renderer/shell_content_renderer_client.h",
     "renderer/shell_render_view_observer.cc",
     "renderer/shell_render_view_observer.h",
     "renderer/test_runner/accessibility_controller.cc",
     "renderer/test_runner/accessibility_controller.h",
     "renderer/test_runner/app_banner_client.cc",
     "renderer/test_runner/app_banner_client.h",
@@ skipping 75 matching lines @@
   deps = [
     ":resources",
     "//base",
     "//base:base_static",
     "//base/allocator",
     "//base/third_party/dynamic_annotations",
     "//cc",
     "//components/crash/app",
     "//components/devtools_discovery",
     "//components/devtools_http_handler",
+    "//components/plugins/renderer",
     "//components/web_cache/renderer",
     "//content:resources",
     "//content/app/resources",
     "//content/app/strings",
     "//content/gpu",
     "//content/test:layouttest_support",
     "//device/bluetooth",
     "//device/bluetooth:mocks",
     "//gin",
     "//gpu",
@@ skipping 304 matching lines @@
 
     deps = [
       "//base",
       "//components/crash/tools:crash_service",
     ]
 
     configs -= [ "//build/config/win:console" ]
     configs += [ "//build/config/win:windowed" ]
   }
 }