Convert enable_plugins to a buildflag header.

chrome/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/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
@@ skipping 1432 matching lines @@
     "//google_apis",
     "//gpu/config",
     "//media",
     "//media/midi",
     "//mojo/common",
     "//mojo/edk/system",
     "//mojo/public/cpp/bindings",
     "//mojo/public/js",
     "//net:extras",
     "//net:net_with_v8",
+    "//ppapi/features",
     "//printing/features",
     "//services/image_decoder/public/cpp",
     "//services/service_manager/public/cpp",
     "//skia",
     "//sql",
     "//storage/browser",
     "//storage/common",
     "//third_party/WebKit/public:features",
     "//third_party/WebKit/public:image_resources",
     "//third_party/WebKit/public:resources",
@@ skipping 2954 matching lines @@
   # linking all of the test support.
   static_library("pepper_cdm_test_constants") {
     testonly = true
     visibility = [ "//chrome/*" ]
     sources = [
       "media/pepper_cdm_test_constants.cc",
       "media/pepper_cdm_test_constants.h",
     ]
   }
 }