Convert enable_plugins to a buildflag header.

content/child/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("//build/config/ui.gni")
+import("//ppapi/features/features.gni")
 import("//third_party/WebKit/public/public_features.gni")
 
 if (is_component_build) {
   link_target_type = "source_set"
 } else {
   link_target_type = "static_library"
 }
 target(link_target_type, "child") {
   # Targets external to content should always link to the public API.
   # Internal targets can link to this but only if they're child processes
@@ skipping 275 matching lines @@
 
 # See comment at the top of //content/BUILD.gn for how this works.
 group("for_content_tests") {
   visibility = [ "//content/test/*" ]
   if (!is_component_build) {
     public_deps = [
       ":child",
     ]
   }
 }