Convert enable_plugins to a buildflag header.

content/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("//tools/grit/grit_rule.gni")
 import("//build/config/nacl/config.gni")
+import("//ppapi/features/features.gni")
 
 # Applied by targets internal to content.
 config("content_implementation") {
   defines = [ "CONTENT_IMPLEMENTATION" ]
   configs = [ "//build/config/compiler:wexit_time_destructors" ]
 }
 
 # When targets depend on, e.g. //content/public/browser, what happens? To
 # facilitate the complexity here, the "public" targets are groups that forward
 # to the right thing depending on the build mode. Say for additional
@@ skipping 128 matching lines @@
     sources = [
       "app/sandbox_helper_win.cc",
       "public/app/sandbox_helper_win.h",
     ]
 
     deps = [
       "//sandbox",
     ]
   }
 }