Convert enable_plugins to a buildflag header.

chrome/common/features.gni

 # Copyright 2015 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/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/features.gni")
 import("//extensions/features/features.gni")
 import("//net/features.gni")
+import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
 
 # Please keep features in alphabetical order.
 declare_args() {
   # Whether the Java UI is being used (Java infobars and popups, Java native
   # settings and first run experience, sign-in etc.).
   # Default to true if compiling for android, but allow this being overriden
   # through the environment.
   android_java_ui = is_android
 
@@ skipping 62 matching lines @@
 
 # Every grit target in //chrome should apply these defines so that the
 # proper build flags can be set.
 chrome_grit_defines = [
   "enable_app_list=$enable_app_list",
   "enable_background=$enable_background",
   "enable_extensions=$enable_extensions",
   "enable_google_now=$enable_google_now",
   "enable_hangout_services_extension=$enable_hangout_services_extension",
   "enable_hotwording=$enable_hotwording",
+  "enable_plugins=$enable_plugins",
   "enable_print_preview=$enable_print_preview",
   "enable_printing=$_enable_printing",
   "enable_service_discovery=$enable_service_discovery",
   "enable_vr_shell=$enable_vr_shell",
   "enable_webvr=$enable_webvr",
   "safe_browsing_mode=$safe_browsing_mode",
   "use_vulcanize=$use_vulcanize",
 ]