Convert enable_plugins to a buildflag header.

components/content_settings/core/browser/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("//ppapi/features/features.gni")
 
 static_library("browser") {
   sources = [
     "content_settings_binary_value_map.cc",
     "content_settings_binary_value_map.h",
     "content_settings_default_provider.cc",
     "content_settings_default_provider.h",
     "content_settings_details.cc",
     "content_settings_details.h",
     "content_settings_info.cc",
@@ skipping 66 matching lines @@
     "//base/test:test_support",
     "//components/content_settings/core/common",
     "//components/content_settings/core/test:test_support",
     "//components/pref_registry:test_support",
     "//components/prefs",
     "//extensions/features",
     "//testing/gtest",
     "//url",
   ]
 }