Move enable_pdf to a buildflag header.

pdf/features.gni

+# 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/config/chromecast_build.gni")
+
+# Most build code won't need to include this file. Instead you can
+# unconditionally depend on "//pdf" which will be a no-op when PDF support is
+# disabled.
+
+declare_args() {
+  enable_pdf = !is_android && !is_ios && !is_chromecast
+}