Convert enable_plugins to a buildflag header.

chrome/renderer/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("//chrome/common/features.gni")
 import("//components/spellcheck/spellcheck_build_features.gni")
 import("//extensions/features/features.gni")
+import("//ppapi/features/features.gni")
 import("//tools/grit/grit_rule.gni")
 
 grit("resources") {
   source = "resources/renderer_resources.grd"
   defines = chrome_grit_defines
   use_qualified_include = true
   output_dir = "$root_gen_dir/chrome"
   output_name = "renderer_resources"
   outputs = [
     "grit/renderer_resources.h",
@@ skipping 371 matching lines @@
     ]
   }
 
   if (is_android) {
     sources -= [
       "safe_browsing/mock_feature_extractor_clock.cc",
       "safe_browsing/mock_feature_extractor_clock.h",
     ]
   }
 }