Convert enable_plugins to a buildflag header.

content/public/common/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/buildflag_header.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
+import("//ppapi/features/features.gni")
 import("//third_party/webrtc/build/webrtc.gni")
 
 # See //content/BUILD.gn for how this works.
 group("common") {
   if (is_component_build) {
     public_deps = [
       "//content",
     ]
   } else {
     public_deps = [
@@ skipping 226 matching lines @@
   ]
   deps = [
     # This looks needless as we have //content/common in public_deps, but it's
     # needed because of allow_circular_includes_from.
     "//content/common:mojo_bindings_cpp_sources",
     "//ipc",
     "//media",
     "//mojo/common",
     "//net",
     "//ppapi/c",
+    "//ppapi/features",
     "//services/service_manager/public/cpp",
     "//skia",
     "//storage/common",
     "//third_party/icu",
     "//ui/accessibility",
     "//ui/base",
     "//ui/gfx",
     "//ui/gfx/ipc",
     "//ui/gfx/ipc/color",
     "//ui/gfx/ipc/geometry",
@@ skipping 42 matching lines @@
   sources = [
     "window_container_type.mojom",
   ]
 }
 
 mojom("service_names") {
   sources = [
     "service_names.mojom",
   ]
 }