Convert enable_plugins to a buildflag header.

content/shell/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/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/config/win/console_app.gni")
 import("//build/config/win/manifest.gni")
 import("//media/media_options.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
+import("//ppapi/features/features.gni")
 import("//tools/grit/repack.gni")
 import("//tools/grit/grit_rule.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
 } else if (is_mac) {
   import("//build/config/mac/rules.gni")
   import("//build/mac/tweak_info_plist.gni")
   import("//third_party/icu/config.gni")
   import("//v8/gni/v8.gni")
 } else if (is_win) {
@@ skipping 219 matching lines @@
     "//device/bluetooth:mocks",
     "//device/geolocation",
     "//gin",
     "//gpu",
     "//ipc",
     "//media",
     "//media/capture",
     "//mojo/edk/js",
     "//net",
     "//net:net_resources",
+    "//ppapi/features",
     "//sandbox",
     "//services/service_manager/public/cpp",
     "//skia",
     "//storage/browser",
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/WebKit/public:blink",
     "//third_party/WebKit/public:features",
     "//third_party/WebKit/public:image_resources",
     "//third_party/WebKit/public:resources",
@@ skipping 488 matching lines @@
       "{{bundle_resources_dir}}/{{source_file_part}}",
     ]
   }
 }
 
 mojom("mojo_bindings") {
   sources = [
     "common/layout_test/layout_test_bluetooth_fake_adapter_setter.mojom",
   ]
 }