Convert enable_plugins to a buildflag header.

chrome/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/util/process_version.gni")
 import("//chrome/common/features.gni")
 import("//chrome/process_version_rc_template.gni")  # For branding_file_path.
 import("//extensions/features/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
+import("//ppapi/features/features.gni")
 import("//tools/grit/grit_rule.gni")
 
 grit("resources") {
   source = "common_resources.grd"
   use_qualified_include = true
   defines = chrome_grit_defines
   output_dir = "$root_gen_dir/chrome"
   output_name = "common_resources"
   outputs = [
     "grit/common_resources.h",
@@ skipping 646 matching lines @@
   sources = [
     "network_diagnostics.mojom",
     "resource_usage_reporter.mojom",
     "shell_handler_win.mojom",
   ]
 
   public_deps = [
     "//url/mojo:url_mojom_gurl",
   ]
 }