Convert enable_plugins to a buildflag header.

content/app/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/chrome_build.gni")
 import("//build/config/features.gni")
+import("//ppapi/features/features.gni")
 
 # Implements "content_main" given the defines and visibility. On Windows this
 # is compiled with a different define for browser and child, but all code needs
 # to be shared.
 template("implement_content_app") {
   runner_target_name = "content_main_runner_" + target_name
   main_target_name = target_name
 
   extra_configs = [
     "//build/config/compiler:wexit_time_destructors",
     "//content:content_implementation",
     "//v8:external_startup_data",
   ]
 
   content_app_deps = [
     "//base",
     "//base:i18n",
     "//components/tracing:startup_tracing",
     "//content:export",
     "//content:sandbox_helper_win",
     "//content/public/common:common_sources",
     "//crypto",
     "//services/service_manager/public/interfaces",
     "//mojo/edk/system",
+    "//ppapi/features",
     "//third_party/WebKit/public:mojo_bindings",
     "//ui/base",
     "//ui/gfx",
     "//ui/gfx/geometry",
   ]
 
   if (is_win) {
     content_app_deps += [ "//sandbox" ]
   } else if (is_android) {
     # Android doesn't use the browser/child split and in the component build
@@ skipping 118 matching lines @@
   implement_content_app("browser") {
     visibility = [ "//content/public/app:browser" ]
     defines = [ "CHROME_MULTIPLE_DLL_BROWSER" ]
   }
 
   implement_content_app("child") {
     visibility = [ "//content/public/app:child" ]
     defines = [ "CHROME_MULTIPLE_DLL_CHILD" ]
   }
 }