Convert enable_plugins to a buildflag header.

content/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/config/features.gni")
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
+import("//ppapi/features/features.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
 if (is_mac) {
   import("//build/config/mac/mac_sdk.gni")
 }
 
 source_set("common") {
   # Targets external to content should always link to the public API.
   # In addition, targets outside of the content component (shell and tests)
   # must not link to this because it will duplicate the code in the component
   # build.
@@ skipping 359 matching lines @@
     "//media/base/ipc",
     "//media/capture",
     "//media/capture/ipc",
     "//media/gpu/ipc/client",
     "//media/gpu/ipc/common",
     "//media/midi",
     "//media/midi:mojo_cpp_sources",
     "//mojo/common:common_base",
     "//mojo/edk/system",
     "//net",
+    "//ppapi/features",
     "//sandbox",
     "//services/service_manager",
     "//services/service_manager/public/cpp",
     "//services/service_manager/public/interfaces",
     "//services/service_manager/runner/common",
     "//services/ui/public/interfaces",
     "//services/video_capture/public/interfaces",
     "//skia",
     "//storage/common",
     "//third_party/WebKit/public:blink_minimal",
@@ skipping 206 matching lines @@
     "//ui/gfx/geometry/mojo",
     "//ui/gfx/mojo",
     "//url/mojo:url_mojom_gurl",
     "//url/mojo:url_mojom_origin",
   ]
 
   export_class_attribute = "CONTENT_EXPORT"
   export_define = "CONTENT_IMPLEMENTATION=1"
   export_header = "content/common/content_export.h"
 }