Convert enable_plugins to a buildflag header.

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")
+import("//ppapi/features/features.gni")
 
 static_library("renderer") {
   sources = [
     "plugin_placeholder.cc",
     "plugin_placeholder.h",
     "webview_plugin.cc",
     "webview_plugin.h",
   ]
   if (enable_plugins) {
     sources += [
       "loadable_plugin_placeholder.cc",
       "loadable_plugin_placeholder.h",
     ]
   }
 
   deps = [
     "//content/public/child",
     "//content/public/common",
     "//content/public/renderer",
     "//gin",
     "//skia",
     "//third_party/WebKit/public:blink",
     "//third_party/re2",
     "//ui/base",
     "//v8",
   ]
 }