Plugin Placeholders: Refactor for platforms that don't support plugins

components/plugins/renderer/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")
+
 static_library("renderer") {
   sources = [
-    "loadable_plugin_placeholder.cc",
-    "loadable_plugin_placeholder.h",
     "plugin_placeholder.cc",
     "plugin_placeholder.h",
     "webview_plugin.cc",
     "webview_plugin.h",
   ]
+  if (enable_plugins) {
+    sources += [
+      "loadable_plugin_placeholder.cc",
+      "loadable_plugin_placeholder.h",
+    ]
+  }
   if (is_android) {
     sources += [
       "mobile_youtube_plugin.cc",
       "mobile_youtube_plugin.h",
     ]
   }
 
   deps = [
     "//gin",
     "//skia",
     "//third_party/WebKit/public:blink",
     "//third_party/re2",
     "//v8",
   ]
 }