Convert enable_plugins to a buildflag header.

ppapi/features/BUILD.gn

 # Copyright 2016 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/buildflag_header.gni")
 import("//ppapi/features/features.gni")
 
 # This file is in a separate directory so all targets in the build can refer to
 # the buildflag header to get the necessary preprocessor defines without
 # bringing in all of Pepper.
 
 buildflag_header("features") {
   header = "features.h"
-  flags = [ "ENABLE_PEPPER_CDMS=$enable_pepper_cdms" ]
+  flags = [
+    "ENABLE_PEPPER_CDMS=$enable_pepper_cdms",
+    "ENABLE_PLUGINS=$enable_plugins",
+  ]
 }