Always use ENABLE_PLUGINS to indicate plugins support.

build/config/features.gni

 # 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.
 
 # This file contains Chrome-feature-related build flags (see ui.gni for
 # UI-related ones). These should theoretically be moved to the build files of
 # the features themselves.
 #
 # However, today we have many "bad" dependencies on some of these flags from,
 # e.g. base, so they need to be global to match the GYP configuration. Also,
 # anything that needs a grit define must be in either this file or ui.gni.
 #
 # PLEASE TRY TO AVOID ADDING FLAGS TO THIS FILE in cases where grit isn't
 # required. See the declare_args block of BUILDCONFIG.gn for advice on how
 # to set up feature flags.
 
 import("//build/config/chrome_build.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 declare_args() {
   # Multicast DNS.
   enable_mdns = is_win || is_linux
 
   enable_plugins = !is_android && !is_ios
+  enable_pdf = !is_android && !is_ios
 
   # Enables Native Client support.
   # TODO(GYP): Get NaCl linking on other platforms.
   # Also, see if we can always get rid of enable_nacl_untrusted and
   # enable_pnacl and always build them if enable_nacl is true.
   # The "is_nacl" part of the condition is needed to ensure that
   # the untrusted code is built properly; arguably it should be
   # guarded by "is_nacl" directly rather than enable_nacl_untrusted, but
   # this will go away when Mac and Win are working and we can just use
   # the commented out logic.
@@ skipping 166 matching lines @@
 # system).
 # TODO(GYP) also require !embedded to enable.
 use_gconf = is_linux && !is_chromeos
 
 # Whether to back up data before sync.
 enable_pre_sync_backup = is_win || is_mac || (is_linux && !is_chromeos)
 
 # Enable WebVR support by default on Android
 # Still requires command line flag to access API
 enable_webvr = is_android