Move enable extensions define to a build flag.

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("//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 = [
@@ skipping 183 matching lines @@
     "//components/strings",
     "//components/translate/content/common",
     "//components/translate/core/common",
     "//components/url_formatter",
     "//components/variations",
     "//components/version_info",
     "//components/visitedlink/common",
     "//content/public/common",
     "//crypto",
     "//extensions/common:common_constants",
+    "//extensions/features",
     "//gin",
     "//google_apis",
     "//gpu/command_buffer/service",
     "//gpu/config",
     "//ipc",
     "//media",
     "//mojo/edk/system",
     "//mojo/public/cpp/bindings",
     "//net",
     "//ppapi/features",
@@ skipping 339 matching lines @@
     "env_vars.h",
     "pref_font_script_names-inl.h",
     "pref_font_webkit_names.h",
     "pref_names.cc",
     "pref_names.h",
   ]
 
   public_deps = [
     ":features",
     "//content/public/common:result_codes",
+    "//extensions/features",
     "//printing/features",
   ]
   deps = [
     ":version_header",
     "//base",
     "//base/third_party/dynamic_annotations",
     "//components/bookmarks/common",
     "//components/nacl/common:switches",
     "//media:cdm_paths",  # Needed by chrome_paths.cc.
     "//ppapi/features",
@@ skipping 83 matching lines @@
   sources = [
     "network_diagnostics.mojom",
     "resource_usage_reporter.mojom",
     "shell_handler_win.mojom",
   ]
 
   public_deps = [
     "//url/mojo:url_mojom_gurl",
   ]
 }